# [c#] How do I calculate someone's age based on a DateTime type birthday?

Given a `DateTime` representing a person's birthday, how do I calculate their age in years?

This question is related to `c#` `.net` `datetime`

## The answer is

I have used the following for this issue. I know it's not very elegant, but it's working.

``````DateTime zeroTime = new DateTime(1, 1, 1);
var date1 = new DateTime(1983, 03, 04);
var date2 = DateTime.Now;
var dif = date2 - date1;
int years = (zeroTime + dif).Year - 1;
Log.DebugFormat("Years -->{0}", years);
``````

``````private int GetYearDiff(DateTime start, DateTime end)
{
int diff = end.Year - start.Year;
if (end.DayOfYear < start.DayOfYear) { diff -= 1; }
return diff;
}
[Fact]
public void GetYearDiff_WhenCalls_ShouldReturnCorrectYearDiff()
{
//arrange
var now = DateTime.Now;
//act
//assert
Assert.Equal(24, GetYearDiff(new DateTime(1992, 7, 9), now)); // passed
Assert.Equal(24, GetYearDiff(new DateTime(1992, now.Month, now.Day), now)); // passed
Assert.Equal(23, GetYearDiff(new DateTime(1992, 12, 9), now)); // passed
}
``````

Keeping it simple (and possibly stupid:)).

``````DateTime birth = new DateTime(1975, 09, 27, 01, 00, 00, 00);
TimeSpan ts = DateTime.Now - birth;
Console.WriteLine("You are approximately " + ts.TotalSeconds.ToString() + " seconds old.");
``````

Here's a little code sample for C# I knocked up, be careful around the edge cases specifically leap years, not all the above solutions take them into account. Pushing the answer out as a DateTime can cause problems as you could end up trying to put too many days into a specific month e.g. 30 days in Feb.

``````public string LoopAge(DateTime myDOB, DateTime FutureDate)
{
int years = 0;
int months = 0;
int days = 0;

DateTime tmpMyDOB = new DateTime(myDOB.Year, myDOB.Month, 1);

DateTime tmpFutureDate = new DateTime(FutureDate.Year, FutureDate.Month, 1);

{
months++;
if (months > 12)
{
years++;
months = months - 12;
}
}

if (FutureDate.Day >= myDOB.Day)
{
days = days + FutureDate.Day - myDOB.Day;
}
else
{
months--;
if (months < 0)
{
years--;
months = months + 12;
}
days = days + (DateTime.DaysInMonth(FutureDate.AddMonths(-1).Year, FutureDate.AddMonths(-1).Month) + FutureDate.Day) - myDOB.Day;

}

//add an extra day if the dob is a leap day
if (DateTime.IsLeapYear(myDOB.Year) && myDOB.Month == 2 && myDOB.Day == 29)
{
//but only if the future date is less than 1st March
if(FutureDate >= new DateTime(FutureDate.Year, 3,1))
days++;
}

return "Years: " + years + " Months: " + months + " Days: " + days;
}
``````

I've spent some time working on this and came up with this to calculate someone's age in years, months and days. I've tested against the Feb 29th problem and leap years and it seems to work, I'd appreciate any feedback:

``````public void LoopAge(DateTime myDOB, DateTime FutureDate)
{
int years = 0;
int months = 0;
int days = 0;

DateTime tmpMyDOB = new DateTime(myDOB.Year, myDOB.Month, 1);

DateTime tmpFutureDate = new DateTime(FutureDate.Year, FutureDate.Month, 1);

{
months++;

if (months > 12)
{
years++;
months = months - 12;
}
}

if (FutureDate.Day >= myDOB.Day)
{
days = days + FutureDate.Day - myDOB.Day;
}
else
{
months--;

if (months < 0)
{
years--;
months = months + 12;
}

days +=
DateTime.DaysInMonth(
) + FutureDate.Day - myDOB.Day;

}

//add an extra day if the dob is a leap day
if (DateTime.IsLeapYear(myDOB.Year) && myDOB.Month == 2 && myDOB.Day == 29)
{
//but only if the future date is less than 1st March
if (FutureDate >= new DateTime(FutureDate.Year, 3, 1))
days++;
}

}
``````

The simplest way I've ever found is this. It works correctly for the US and western europe locales. Can't speak to other locales, especially places like China. 4 extra compares, at most, following the initial computation of age.

``````public int AgeInYears(DateTime birthDate, DateTime referenceDate)
{
Debug.Assert(referenceDate >= birthDate,
"birth date must be on or prior to the reference date");

DateTime birth = birthDate.Date;
DateTime reference = referenceDate.Date;
int years = (reference.Year - birth.Year);

//
// an offset of -1 is applied if the birth date has
// not yet occurred in the current year.
//
if (reference.Month > birth.Month);
else if (reference.Month < birth.Month)
--years;
else // in birth month
{
if (reference.Day < birth.Day)
--years;
}

return years ;
}
``````

I was looking over the answers to this and noticed that nobody has made reference to regulatory/legal implications of leap day births. For instance, per Wikipedia, if you're born on February 29th in various jurisdictions, you're non-leap year birthday varies:

• In the United Kingdom and Hong Kong: it's the ordinal day of the year, so the next day, March 1st is your birthday.
• In New Zealand: it's the previous day, February 28th for the purposes of driver licencing, and March 1st for other purposes.
• Taiwan: it's February 28th.

And as near as I can tell, in the US, the statutes are silent on the matter, leaving it up to the common law and to how various regulatory bodies define things in their regulations.

To that end, an improvement:

``````public enum LeapDayRule
{
OrdinalDay     = 1 ,
LastDayOfMonth = 2 ,
}

static int ComputeAgeInYears(DateTime birth, DateTime reference, LeapYearBirthdayRule ruleInEffect)
{
bool isLeapYearBirthday = CultureInfo.CurrentCulture.Calendar.IsLeapDay(birth.Year, birth.Month, birth.Day);
DateTime cutoff;

if (isLeapYearBirthday && !DateTime.IsLeapYear(reference.Year))
{
switch (ruleInEffect)
{
case LeapDayRule.OrdinalDay:
cutoff = new DateTime(reference.Year, 1, 1)
break;

case LeapDayRule.LastDayOfMonth:
cutoff = new DateTime(reference.Year, birth.Month, 1)
break;

default:
throw new InvalidOperationException();
}
}
else
{
cutoff = new DateTime(reference.Year, birth.Month, birth.Day);
}

int age = (reference.Year - birth.Year) + (reference >= cutoff ? 0 : -1);
return age < 0 ? 0 : age;
}
``````

It should be noted that this code assumes:

• A western (European) reckoning of age, and
• A calendar, like the Gregorian calendar that inserts a single leap day at the end of a month.

I think the TimeSpan has all that we need in it, without having to resort to 365.25 (or any other approximation). Expanding on Aug's example:

``````DateTime myBD = new DateTime(1980, 10, 10);
TimeSpan difference = DateTime.Now.Subtract(myBD);

textBox1.Text = difference.Years + " years " + difference.Months + " Months " + difference.Days + " days";
``````

Here's yet another answer:

``````public static int AgeInYears(DateTime birthday, DateTime today)
{
return ((today.Year - birthday.Year) * 372 + (today.Month - birthday.Month) * 31 + (today.Day - birthday.Day)) / 372;
}
``````

This has been extensively unit-tested. It does look a bit "magic". The number 372 is the number of days there would be in a year if every month had 31 days.

The explanation of why it works (lifted from here) is:

Let's set `Yn = DateTime.Now.Year, Yb = birthday.Year, Mn = DateTime.Now.Month, Mb = birthday.Month, Dn = DateTime.Now.Day, Db = birthday.Day`

`age = Yn - Yb + (31*(Mn - Mb) + (Dn - Db)) / 372`

We know that what we need is either `Yn-Yb` if the date has already been reached, `Yn-Yb-1` if it has not.

a) If `Mn<Mb`, we have `-341 <= 31*(Mn-Mb) <= -31 and -30 <= Dn-Db <= 30`

`-371 <= 31*(Mn - Mb) + (Dn - Db) <= -1`

With integer division

`(31*(Mn - Mb) + (Dn - Db)) / 372 = -1`

b) If `Mn=Mb` and `Dn<Db`, we have `31*(Mn - Mb) = 0 and -30 <= Dn-Db <= -1`

With integer division, again

`(31*(Mn - Mb) + (Dn - Db)) / 372 = -1`

c) If `Mn>Mb`, we have `31 <= 31*(Mn-Mb) <= 341 and -30 <= Dn-Db <= 30`

`1 <= 31*(Mn - Mb) + (Dn - Db) <= 371`

With integer division

`(31*(Mn - Mb) + (Dn - Db)) / 372 = 0`

d) If `Mn=Mb` and `Dn>Db`, we have `31*(Mn - Mb) = 0 and 1 <= Dn-Db <= 3`0

With integer division, again

`(31*(Mn - Mb) + (Dn - Db)) / 372 = 0`

e) If `Mn=Mb` and `Dn=Db`, we have `31*(Mn - Mb) + Dn-Db = 0`

and therefore `(31*(Mn - Mb) + (Dn - Db)) / 372 = 0`

2 Main problems to solve are:

1. Calculate Exact age - in years, months, days, etc.

2. Calculate Generally perceived age - people usually do not care how old they exactly are, they just care when their birthday in the current year is.

Solution for 1 is obvious:

``````DateTime birth = DateTime.Parse("1.1.2000");
DateTime today = DateTime.Today;     //we usually don't care about birth time
TimeSpan age = today - birth;        //.NET FCL should guarantee this as precise
double ageInDays = age.TotalDays;    //total number of days ... also precise
double daysInYear = 365.2425;        //statistical value for 400 years
double ageInYears = ageInDays / daysInYear;  //can be shifted ... not so precise
``````

Solution for 2 is the one which is not so precise in determing total age, but is perceived as precise by people. People also usually use it, when they calculate their age "manually":

``````DateTime birth = DateTime.Parse("1.1.2000");
DateTime today = DateTime.Today;
int age = today.Year - birth.Year;    //people perceive their age in years

if (today.Month < birth.Month ||
((today.Month == birth.Month) && (today.Day < birth.Day)))
{
age--;  //birthday in current year not yet reached, we are 1 year younger ;)
//+ no birthday for 29.2. guys ... sorry, just wrong date for birth
}
``````

Notes to 2.:

• This is my preferred solution
• We cannot use DateTime.DayOfYear or TimeSpans, as they shift number of days in leap years
• I have put there little more lines for readability

Just one more note ... I would create 2 static overloaded methods for it, one for universal usage, second for usage-friendliness:

``````public static int GetAge(DateTime bithDay, DateTime today)
{
//chosen solution method body
}

public static int GetAge(DateTime birthDay)
{
return GetAge(birthDay, DateTime.Now);
}
``````

I have created a SQL Server User Defined Function to calculate someone's age, given their birthdate. This is useful when you need it as part of a query:

``````using System;
using System.Data;
using System.Data.Sql;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;

public partial class UserDefinedFunctions
{
public static SqlInt32 CalculateAge(string strBirthDate)
{
DateTime dtBirthDate = new DateTime();
dtBirthDate = Convert.ToDateTime(strBirthDate);
DateTime dtToday = DateTime.Now;

// get the difference in years
int years = dtToday.Year - dtBirthDate.Year;

// subtract another year if we're before the
// birth day in the current year
if (dtToday.Month < dtBirthDate.Month || (dtToday.Month == dtBirthDate.Month && dtToday.Day < dtBirthDate.Day))
years=years-1;

int intCustomerAge = years;
return intCustomerAge;
}
};
``````

I think this problem can be solved with an easier way like this-

The class can be like-

``````using System;

namespace TSA
{
class BirthDay
{
double ageDay;
public BirthDay(int day, int month, int year)
{
DateTime birthDate = new DateTime(year, month, day);
ageDay = (birthDate - DateTime.Now).TotalDays; //DateTime.UtcNow
}

internal int GetAgeYear()
{
return (int)Math.Truncate(ageDay / 365);
}

internal int GetAgeMonth()
{
return (int)Math.Truncate((ageDay % 365) / 30);
}
}
}
``````

And calls can be like-

``````BirthDay b = new BirthDay(1,12,1990);
int year = b.GetAgeYear();
int month = b.GetAgeMonth();
``````

``````var birthDate = ... // DOB
var resultDate = DateTime.Now - birthDate;
``````

Using `resultDate` you can apply `TimeSpan` properties whatever you want to display it.

Here is a very simple and easy to follow example.

``````private int CalculateAge()
{
//get birthdate
DateTime dtBirth = Convert.ToDateTime(BirthDatePicker.Value);
int byear = dtBirth.Year;
int bmonth = dtBirth.Month;
int bday = dtBirth.Day;
DateTime dtToday = DateTime.Now;
int tYear = dtToday.Year;
int tmonth = dtToday.Month;
int tday = dtToday.Day;
int age = tYear - byear;
if (bmonth < tmonth)
age--;
else if (bmonth == tmonth && bday>tday)
{
age--;
}
return age;
}
``````

SQL version:

``````declare @dd smalldatetime = '1980-04-01'
declare @age int = YEAR(GETDATE())-YEAR(@dd)
if (@dd> DATEADD(YYYY, -@age, GETDATE())) set @age = @age -1

print @age
``````

With fewer conversions and UtcNow, this code can take care of someone born on the Feb 29 in a leap year:

``````public int GetAge(DateTime DateOfBirth)
{
var Now = DateTime.UtcNow;
return Now.Year - DateOfBirth.Year -
(
(
Now.Month > DateOfBirth.Month ||
(Now.Month == DateOfBirth.Month && Now.Day >= DateOfBirth.Day)
) ? 0 : 1
);
}
``````

Try this solution, it's working.

``````int age = (Int32.Parse(DateTime.Today.ToString("yyyyMMdd")) -
Int32.Parse(birthday.ToString("yyyyMMdd rawrrr"))) / 10000;
``````

The simple answer to this is to apply `AddYears` as shown below because this is the only native method to add years to the 29th of Feb. of leap years and obtain the correct result of the 28th of Feb. for common years.

Some feel that 1th of Mar. is the birthday of leaplings but neither .Net nor any official rule supports this, nor does common logic explain why some born in February should have 75% of their birthdays in another month.

Further, an Age method lends itself to be added as an extension to `DateTime`. By this you can obtain the age in the simplest possible way:

1. List item

int age = birthDate.Age();

``````public static class DateTimeExtensions
{
/// <summary>
/// Calculates the age in years of the current System.DateTime object today.
/// </summary>
/// <param name="birthDate">The date of birth</param>
/// <returns>Age in years today. 0 is returned for a future date of birth.</returns>
public static int Age(this DateTime birthDate)
{
return Age(birthDate, DateTime.Today);
}

/// <summary>
/// Calculates the age in years of the current System.DateTime object on a later date.
/// </summary>
/// <param name="birthDate">The date of birth</param>
/// <param name="laterDate">The date on which to calculate the age.</param>
/// <returns>Age in years on a later day. 0 is returned as minimum.</returns>
public static int Age(this DateTime birthDate, DateTime laterDate)
{
int age;
age = laterDate.Year - birthDate.Year;

if (age > 0)
{
age -= Convert.ToInt32(laterDate.Date < birthDate.Date.AddYears(age));
}
else
{
age = 0;
}

return age;
}
}
``````

Now, run this test:

``````class Program
{
static void Main(string[] args)
{
RunTest();
}

private static void RunTest()
{
DateTime birthDate = new DateTime(2000, 2, 28);
DateTime laterDate = new DateTime(2011, 2, 27);
string iso = "yyyy-MM-dd";

for (int i = 0; i < 3; i++)
{
for (int j = 0; j < 3; j++)
{
Console.WriteLine("Birth date: " + birthDate.AddDays(i).ToString(iso) + "  Later date: " + laterDate.AddDays(j).ToString(iso) + "  Age: " + birthDate.AddDays(i).Age(laterDate.AddDays(j)).ToString());
}
}

}
}
``````

The critical date example is this:

Birth date: 2000-02-29 Later date: 2011-02-28 Age: 11

Output:

``````{
Birth date: 2000-02-28  Later date: 2011-02-27  Age: 10
Birth date: 2000-02-28  Later date: 2011-02-28  Age: 11
Birth date: 2000-02-28  Later date: 2011-03-01  Age: 11
Birth date: 2000-02-29  Later date: 2011-02-27  Age: 10
Birth date: 2000-02-29  Later date: 2011-02-28  Age: 11
Birth date: 2000-02-29  Later date: 2011-03-01  Age: 11
Birth date: 2000-03-01  Later date: 2011-02-27  Age: 10
Birth date: 2000-03-01  Later date: 2011-02-28  Age: 10
Birth date: 2000-03-01  Later date: 2011-03-01  Age: 11
}
``````

And for the later date 2012-02-28:

``````{
Birth date: 2000-02-28  Later date: 2012-02-28  Age: 12
Birth date: 2000-02-28  Later date: 2012-02-29  Age: 12
Birth date: 2000-02-28  Later date: 2012-03-01  Age: 12
Birth date: 2000-02-29  Later date: 2012-02-28  Age: 11
Birth date: 2000-02-29  Later date: 2012-02-29  Age: 12
Birth date: 2000-02-29  Later date: 2012-03-01  Age: 12
Birth date: 2000-03-01  Later date: 2012-02-28  Age: 11
Birth date: 2000-03-01  Later date: 2012-02-29  Age: 11
Birth date: 2000-03-01  Later date: 2012-03-01  Age: 12
}
``````

I've created an Age struct, which looks like this:

``````public struct Age : IEquatable<Age>, IComparable<Age>
{
private readonly int _years;
private readonly int _months;
private readonly int _days;

public int Years  { get { return _years; } }
public int Months { get { return _months; } }
public int Days { get { return _days; } }

public Age( int years, int months, int days ) : this()
{
_years = years;
_months = months;
_days = days;
}

public static Age CalculateAge( DateTime dateOfBirth, DateTime date )
{
// Here is some logic that ressembles Mike's solution, although it
// also takes into account months & days.
// Ommitted for brevity.
return new Age (years, months, days);
}

// Ommited Equality, Comparable, GetHashCode, functionality for brevity.
}
``````

Here's a one-liner:

``````int age = new DateTime(DateTime.Now.Subtract(birthday).Ticks).Year-1;
``````

``````public string GetAge(this DateTime birthdate, string ageStrinFormat = null)
{
return string.Format(ageStrinFormat ?? "{0}/{1}/{2}",
(date.Year - birthdate.Year), date.Month, date.Day);
}
``````

``````int Age = new DateTime((DateTime.Now - BirthDate).Ticks).Year -1;
Console.WriteLine("Age {0}", Age);
``````

=== Common Saying (from months to years old) ===

If you just for common use, here is the code as your information:

``````DateTime today = DateTime.Today;
DateTime bday = DateTime.Parse("2016-2-14");
int age = today.Year - bday.Year;
var unit = "";

if (bday > today.AddYears(-age))
{
age--;
}
if (age == 0)   // Under one year old
{
age = today.Month - bday.Month;

age = age <= 0 ? (12 + age) : age;  // The next year before birthday

age = today.Day - bday.Day >= 0 ? age : --age;  // Before the birthday.day

unit = "month";
}
else {
unit = "year";
}

if (age > 1)
{
unit = unit + "s";
}
``````

The test result as below:

``````The birthday: 2016-2-14

2016-2-15 =>  age=0, unit=month;
2016-5-13 =>  age=2, unit=months;
2016-5-14 =>  age=3, unit=months;
2016-6-13 =>  age=3, unit=months;
2016-6-15 =>  age=4, unit=months;
2017-1-13 =>  age=10, unit=months;
2017-1-14 =>  age=11, unit=months;
2017-2-13 =>  age=11, unit=months;
2017-2-14 =>  age=1, unit=year;
2017-2-15 =>  age=1, unit=year;
2017-3-13 =>  age=1, unit=year;
2018-1-13 =>  age=1, unit=year;
2018-1-14 =>  age=1, unit=year;
2018-2-13 =>  age=1, unit=year;
2018-2-14 =>  age=2, unit=years;
``````

It can be this simple:

``````int age = DateTime.Now.AddTicks(0 - dob.Ticks).Year - 1;
``````

Here is the simplest way to calculate someone's age.
Calculating someone's age is pretty straightforward, and here's how! In order for the code to work, you need a DateTime object called BirthDate containing the birthday.

`````` C#
// get the difference in years
int years = DateTime.Now.Year - BirthDate.Year;
// subtract another year if we're before the
// birth day in the current year
if (DateTime.Now.Month < BirthDate.Month ||
(DateTime.Now.Month == BirthDate.Month &&
DateTime.Now.Day < BirthDate.Day))
years--;
VB.NET
' get the difference in years
Dim years As Integer = DateTime.Now.Year - BirthDate.Year
' subtract another year if we're before the
' birth day in the current year
If DateTime.Now.Month < BirthDate.Month Or (DateTime.Now.Month = BirthDate.Month And DateTime.Now.Day < BirthDate.Day) Then
years = years - 1
End If
``````

The best way that I know of because of leap years and everything is:

``````DateTime birthDate = new DateTime(2000,3,1);
int age = (int)Math.Floor((DateTime.Now - birthDate).TotalDays / 365.25D);
``````

This may work:

``````public override bool IsValid(DateTime value)
{
_dateOfBirth =  value;
var yearsOld = (double) (DateTime.Now.Subtract(_dateOfBirth).TotalDays/365);
if (yearsOld > 18)
return true;
return false;
}
``````

This is a strange way to do it, but if you format the date to `yyyymmdd` and subtract the date of birth from the current date then drop the last 4 digits you've got the age :)

I don't know C#, but I believe this will work in any language.

``````20080814 - 19800703 = 280111
``````

Drop the last 4 digits = `28`.

C# Code:

``````int now = int.Parse(DateTime.Now.ToString("yyyyMMdd"));
int dob = int.Parse(dateOfBirth.ToString("yyyyMMdd"));
int age = (now - dob) / 10000;
``````

Or alternatively without all the type conversion in the form of an extension method. Error checking omitted:

``````public static Int32 GetAge(this DateTime dateOfBirth)
{
var today = DateTime.Today;

var a = (today.Year * 100 + today.Month) * 100 + today.Day;
var b = (dateOfBirth.Year * 100 + dateOfBirth.Month) * 100 + dateOfBirth.Day;

return (a - b) / 10000;
}
``````

I want to add Hebrew calendar calculations (or other System.Globalization calendar can be used in the same way), using rewrited functions from this thread:

``````Public Shared Function CalculateAge(BirthDate As DateTime) As Integer
Dim HebCal As New System.Globalization.HebrewCalendar ()
Dim now = DateTime.Now()
Dim iAge = HebCal.GetYear(now) - HebCal.GetYear(BirthDate)
Dim iNowMonth = HebCal.GetMonth(now), iBirthMonth = HebCal.GetMonth(BirthDate)
If iNowMonth < iBirthMonth Or (iNowMonth = iBirthMonth AndAlso HebCal.GetDayOfMonth(now) < HebCal.GetDayOfMonth(BirthDate)) Then iAge -= 1
Return iAge
End Function
``````

Here is a function that is serving me well. No calculations , very simple.

``````    public static string ToAge(this DateTime dob, DateTime? toDate = null)
{
if (!toDate.HasValue)
toDate = DateTime.Now;
var now = toDate.Value;

if (now.CompareTo(dob) < 0)
return "Future date";

int years = now.Year - dob.Year;
int months = now.Month - dob.Month;
int days = now.Day - dob.Day;

if (days < 0)
{
months--;
days = DateTime.DaysInMonth(dob.Year, dob.Month) - dob.Day + now.Day;
}

if (months < 0)
{
years--;
months = 12 + months;
}

return string.Format("{0} year(s), {1} month(s), {2} days(s)",
years,
months,
days);
}
``````

And here is a unit test:

``````    [Test]
public void ToAgeTests()
{
var date = new DateTime(2000, 1, 1);
Assert.AreEqual("0 year(s), 0 month(s), 1 days(s)", new DateTime(1999, 12, 31).ToAge(date));
Assert.AreEqual("0 year(s), 0 month(s), 0 days(s)", new DateTime(2000, 1, 1).ToAge(date));
Assert.AreEqual("1 year(s), 0 month(s), 0 days(s)", new DateTime(1999, 1, 1).ToAge(date));
Assert.AreEqual("0 year(s), 11 month(s), 0 days(s)", new DateTime(1999, 2, 1).ToAge(date));
Assert.AreEqual("0 year(s), 10 month(s), 25 days(s)", new DateTime(1999, 2, 4).ToAge(date));
Assert.AreEqual("0 year(s), 10 month(s), 1 days(s)", new DateTime(1999, 2, 28).ToAge(date));

date = new DateTime(2000, 2, 15);
Assert.AreEqual("0 year(s), 0 month(s), 28 days(s)", new DateTime(2000, 1, 18).ToAge(date));
}
``````

This classic question is deserving of a Noda Time solution.

``````static int GetAge(LocalDate dateOfBirth)
{
Instant now = SystemClock.Instance.Now;

// The target time zone is important.
// It should align with the *current physical location* of the person
// you are talking about.  When the whereabouts of that person are unknown,
// then you use the time zone of the person who is *asking* for the age.
// The time zone of birth is irrelevant!

DateTimeZone zone = DateTimeZoneProviders.Tzdb["America/New_York"];

LocalDate today = now.InZone(zone).Date;

Period period = Period.Between(dateOfBirth, today, PeriodUnits.Years);

return (int) period.Years;
}
``````

Usage:

``````LocalDate dateOfBirth = new LocalDate(1976, 8, 27);
int age = GetAge(dateOfBirth);
``````

You might also be interested in the following improvements:

• Passing in the clock as an `IClock`, instead of using `SystemClock.Instance`, would improve testability.

• The target time zone will likely change, so you'd want a `DateTimeZone` parameter as well.

See also my blog post on this subject: Handling Birthdays, and Other Anniversaries

This is not a direct answer, but more of a philosophical reasoning about the problem at hand from a quasi-scientific point of view.

I would argue that the question does not specify the unit nor culture in which to measure age, most answers seem to assume an integer annual representation. The SI-unit for time is `second`, ergo the correct generic answer should be (of course assuming normalized `DateTime` and taking no regard whatsoever to relativistic effects):

``````var lifeInSeconds = (DateTime.Now.Ticks - then.Ticks)/TickFactor;
``````

In the Christian way of calculating age in years:

``````var then = ... // Then, in this case the birthday
var now = DateTime.UtcNow;
int age = now.Year - then.Year;
if (now.AddYears(-age) < then) age--;
``````

In finance there is a similar problem when calculating something often referred to as the Day Count Fraction, which roughly is a number of years for a given period. And the age issue is really a time measuring issue.

Example for the actual/actual (counting all days "correctly") convention:

``````DateTime start, end = .... // Whatever, assume start is before end

double startYearContribution = 1 - (double) start.DayOfYear / (double) (DateTime.IsLeapYear(start.Year) ? 366 : 365);
double endYearContribution = (double)end.DayOfYear / (double)(DateTime.IsLeapYear(end.Year) ? 366 : 365);
double middleContribution = (double) (end.Year - start.Year - 1);

double DCF = startYearContribution + endYearContribution + middleContribution;
``````

Another quite common way to measure time generally is by "serializing" (the dude who named this date convention must seriously have been trippin'):

``````DateTime start, end = .... // Whatever, assume start is before end
int days = (end - start).Days;
``````

I wonder how long we have to go before a relativistic age in seconds becomes more useful than the rough approximation of earth-around-sun-cycles during one's lifetime so far :) Or in other words, when a period must be given a location or a function representing motion for itself to be valid :)

Do we need to consider people who is smaller than 1 year? as Chinese culture, we describe small babies' age as 2 months or 4 weeks.

Below is my implementation, it is not as simple as what I imagined, especially to deal with date like 2/28.

``````public static string HowOld(DateTime birthday, DateTime now)
{
if (now < birthday)
throw new ArgumentOutOfRangeException("birthday must be less than now.");

TimeSpan diff = now - birthday;
int diffDays = (int)diff.TotalDays;

if (diffDays > 7)//year, month and week
{
int age = now.Year - birthday.Year;

if (birthday > now.AddYears(-age))
age--;

if (age > 0)
{
return age + (age > 1 ? " years" : " year");
}
else
{// month and week
DateTime d = birthday;
int diffMonth = 1;

while (d.AddMonths(diffMonth) <= now)
{
diffMonth++;
}

age = diffMonth-1;

if (age == 1 && d.Day > now.Day)
age--;

if (age > 0)
{
return age + (age > 1 ? " months" : " month");
}
else
{
age = diffDays / 7;
return age + (age > 1 ? " weeks" : " week");
}
}
}
else if (diffDays > 0)
{
int age = diffDays;
return age + (age > 1 ? " days" : " day");
}
else
{
int age = diffDays;
return "just born";
}
}
``````

This implementation has passed below test cases.

``````[TestMethod]
public void TestAge()
{
string age = HowOld(new DateTime(2011, 1, 1), new DateTime(2012, 11, 30));
Assert.AreEqual("1 year", age);

age = HowOld(new DateTime(2011, 11, 30), new DateTime(2012, 11, 30));
Assert.AreEqual("1 year", age);

age = HowOld(new DateTime(2001, 1, 1), new DateTime(2012, 11, 30));
Assert.AreEqual("11 years", age);

age = HowOld(new DateTime(2012, 1, 1), new DateTime(2012, 11, 30));
Assert.AreEqual("10 months", age);

age = HowOld(new DateTime(2011, 12, 1), new DateTime(2012, 11, 30));
Assert.AreEqual("11 months", age);

age = HowOld(new DateTime(2012, 10, 1), new DateTime(2012, 11, 30));
Assert.AreEqual("1 month", age);

age = HowOld(new DateTime(2008, 2, 28), new DateTime(2009, 2, 28));
Assert.AreEqual("1 year", age);

age = HowOld(new DateTime(2008, 3, 28), new DateTime(2009, 2, 28));
Assert.AreEqual("11 months", age);

age = HowOld(new DateTime(2008, 3, 28), new DateTime(2009, 3, 28));
Assert.AreEqual("1 year", age);

age = HowOld(new DateTime(2009, 1, 28), new DateTime(2009, 2, 28));
Assert.AreEqual("1 month", age);

age = HowOld(new DateTime(2009, 2, 1), new DateTime(2009, 3, 1));
Assert.AreEqual("1 month", age);

// NOTE.
// new DateTime(2008, 1, 31).AddMonths(1) == new DateTime(2009, 2, 28);
// new DateTime(2008, 1, 28).AddMonths(1) == new DateTime(2009, 2, 28);
age = HowOld(new DateTime(2009, 1, 31), new DateTime(2009, 2, 28));
Assert.AreEqual("4 weeks", age);

age = HowOld(new DateTime(2009, 2, 1), new DateTime(2009, 2, 28));
Assert.AreEqual("3 weeks", age);

age = HowOld(new DateTime(2009, 2, 1), new DateTime(2009, 3, 1));
Assert.AreEqual("1 month", age);

age = HowOld(new DateTime(2012, 11, 5), new DateTime(2012, 11, 30));
Assert.AreEqual("3 weeks", age);

age = HowOld(new DateTime(2012, 11, 1), new DateTime(2012, 11, 30));
Assert.AreEqual("4 weeks", age);

age = HowOld(new DateTime(2012, 11, 20), new DateTime(2012, 11, 30));
Assert.AreEqual("1 week", age);

age = HowOld(new DateTime(2012, 11, 25), new DateTime(2012, 11, 30));
Assert.AreEqual("5 days", age);

age = HowOld(new DateTime(2012, 11, 29), new DateTime(2012, 11, 30));
Assert.AreEqual("1 day", age);

age = HowOld(new DateTime(2012, 11, 30), new DateTime(2012, 11, 30));
Assert.AreEqual("just born", age);

age = HowOld(new DateTime(2000, 2, 29), new DateTime(2009, 2, 28));
Assert.AreEqual("8 years", age);

age = HowOld(new DateTime(2000, 2, 29), new DateTime(2009, 3, 1));
Assert.AreEqual("9 years", age);

Exception e = null;

try
{
age = HowOld(new DateTime(2012, 12, 1), new DateTime(2012, 11, 30));
}
catch (ArgumentOutOfRangeException ex)
{
e = ex;
}

Assert.IsTrue(e != null);
}
``````

This is the easiest way to answer this in a single line.

``````DateTime Dob = DateTime.Parse("1985-04-24");

int Age = DateTime.MinValue.AddDays(DateTime.Now.Subtract(Dob).TotalHours/24 - 1).Year - 1;
``````

This also works for leap years.

Just use:

``````(DateTime.Now - myDate).TotalHours / 8766.0
``````

The current date - `myDate = TimeSpan`, get total hours and divide in the total hours per year and get exactly the age/months/days...

``````private int GetAge(int _year, int _month, int _day
{
DateTime yourBirthDate= new DateTime(_year, _month, _day);

DateTime todaysDateTime = DateTime.Today;
int noOfYears = todaysDateTime.Year - yourBirthDate.Year;

if (DateTime.Now.Month < yourBirthDate.Month ||
(DateTime.Now.Month == yourBirthDate.Month && DateTime.Now.Day < yourBirthDate.Day))
{
noOfYears--;
}

return  noOfYears;
}
``````

Another function, not by me but found on the web and refined it a bit:

``````public static int GetAge(DateTime birthDate)
{
DateTime n = DateTime.Now; // To avoid a race condition around midnight
int age = n.Year - birthDate.Year;

if (n.Month < birthDate.Month || (n.Month == birthDate.Month && n.Day < birthDate.Day))
age--;

return age;
}
``````

Just two things that come into my mind: What about people from countries that do not use the Gregorian calendar? DateTime.Now is in the server-specific culture I think. I have absolutely zero knowledge about actually working with Asian calendars and I do not know if there is an easy way to convert dates between calendars, but just in case you're wondering about those Chinese guys from the year 4660 :-)

``````static string CalcAge(DateTime birthDay)
{
DateTime currentDate = DateTime.Now;
int approximateAge = currentDate.Year - birthDay.Year;
int daysToNextBirthDay = (birthDay.Month * 30 + birthDay.Day) -
(currentDate.Month * 30 + currentDate.Day) ;

if (approximateAge == 0 || approximateAge == 1)
{
int month =  Math.Abs(daysToNextBirthDay / 30);
int days = Math.Abs(daysToNextBirthDay % 30);

if (month == 0)
return "Your age is: " + daysToNextBirthDay + " days";

return "Your age is: " + month + " months and " + days + " days"; ;
}

if (daysToNextBirthDay > 0)
return "Your age is: " + --approximateAge + " Years";

return "Your age is: " + approximateAge + " Years"; ;
}
``````

This is simple and appears to be accurate for my needs. I am making an assumption for the purpose of leap years that regardless of when the person chooses to celebrate the birthday they are not technically a year older until 365 days have passed since their last birthday (i.e 28th February does not make them a year older).

``````DateTime now = DateTime.Today;
DateTime birthday = new DateTime(1991, 02, 03);//3rd feb

int age = now.Year - birthday.Year;

if (now.Month < birthday.Month || (now.Month == birthday.Month && now.Day < birthday.Day))//not had bday this year yet
age--;

return age;
``````

I used ScArcher2's solution for an accurate Year calculation of a persons age but I needed to take it further and calculate their Months and Days along with the Years.

``````    public static Dictionary<string,int> CurrentAgeInYearsMonthsDays(DateTime? ndtBirthDate, DateTime? ndtReferralDate)
{
//----------------------------------------------------------------------
// Can't determine age if we don't have a dates.
//----------------------------------------------------------------------
if (ndtBirthDate == null) return null;
if (ndtReferralDate == null) return null;

DateTime dtBirthDate = Convert.ToDateTime(ndtBirthDate);
DateTime dtReferralDate = Convert.ToDateTime(ndtReferralDate);

//----------------------------------------------------------------------
// Create our Variables
//----------------------------------------------------------------------
Dictionary<string, int> dYMD = new Dictionary<string,int>();
int iNowDate, iBirthDate, iYears, iMonths, iDays;
string sDif = "";

//----------------------------------------------------------------------
// Store off current date/time and DOB into local variables
//----------------------------------------------------------------------
iNowDate = int.Parse(dtReferralDate.ToString("yyyyMMdd"));
iBirthDate = int.Parse(dtBirthDate.ToString("yyyyMMdd"));

//----------------------------------------------------------------------
// Calculate Years
//----------------------------------------------------------------------
sDif = (iNowDate - iBirthDate).ToString();
iYears = int.Parse(sDif.Substring(0, sDif.Length - 4));

//----------------------------------------------------------------------
// Store Years in Return Value
//----------------------------------------------------------------------

//----------------------------------------------------------------------
// Calculate Months
//----------------------------------------------------------------------
if (dtBirthDate.Month > dtReferralDate.Month)
iMonths = 12 - dtBirthDate.Month + dtReferralDate.Month - 1;
else
iMonths = dtBirthDate.Month - dtReferralDate.Month;

//----------------------------------------------------------------------
// Store Months in Return Value
//----------------------------------------------------------------------

//----------------------------------------------------------------------
// Calculate Remaining Days
//----------------------------------------------------------------------
if (dtBirthDate.Day > dtReferralDate.Day)
//Logic: Figure out the days in month previous to the current month, or the admitted month.
//       Subtract the birthday from the total days which will give us how many days the person has lived since their birthdate day the previous month.
//       then take the referral date and simply add the number of days the person has lived this month.

//If referral date is january, we need to go back to the following year's December to get the days in that month.
if (dtReferralDate.Month == 1)
iDays = DateTime.DaysInMonth(dtReferralDate.Year - 1, 12) - dtBirthDate.Day + dtReferralDate.Day;
else
iDays = DateTime.DaysInMonth(dtReferralDate.Year, dtReferralDate.Month - 1) - dtBirthDate.Day + dtReferralDate.Day;
else
iDays = dtReferralDate.Day - dtBirthDate.Day;

//----------------------------------------------------------------------
// Store Days in Return Value
//----------------------------------------------------------------------

return dYMD;
}
``````

I've made one small change to Mark Soen's answer: I've rewriten the third line so that the expression can be parsed a bit more easily.

``````public int AgeInYears(DateTime bday)
{
DateTime now = DateTime.Today;
int age = now.Year - bday.Year;
if (bday.AddYears(age) > now)
age--;
return age;
}
``````

I've also made it into a function for the sake of clarity.

Here is a test snippet:

``````DateTime bDay = new DateTime(2000, 2, 29);
DateTime now = new DateTime(2009, 2, 28);
MessageBox.Show(string.Format("Test {0} {1} {2}",
CalculateAgeWrong1(bDay, now),      // outputs 9
CalculateAgeWrong2(bDay, now),      // outputs 9
CalculateAgeCorrect(bDay, now),     // outputs 8
CalculateAgeCorrect2(bDay, now)));  // outputs 8
``````

Here you have the methods:

``````public int CalculateAgeWrong1(DateTime birthDate, DateTime now)
{
return new DateTime(now.Subtract(birthDate).Ticks).Year - 1;
}

public int CalculateAgeWrong2(DateTime birthDate, DateTime now)
{
int age = now.Year - birthDate.Year;

if (now < birthDate.AddYears(age))
age--;

return age;
}

public int CalculateAgeCorrect(DateTime birthDate, DateTime now)
{
int age = now.Year - birthDate.Year;

if (now.Month < birthDate.Month || (now.Month == birthDate.Month && now.Day < birthDate.Day))
age--;

return age;
}

public int CalculateAgeCorrect2(DateTime birthDate, DateTime now)
{
int age = now.Year - birthDate.Year;

// For leap years we need this
if (birthDate > now.AddYears(-age))
age--;
// Don't use:
// if (birthDate.AddYears(age) > now)
//     age--;

return age;
}
``````

``````TimeSpan diff = DateTime.Now - birthdayDateTime;
string age = String.Format("{0:%y} years, {0:%M} months, {0:%d}, days old", diff);
``````

I'm not sure how exactly you'd like it returned to you, so I just made a readable string.

Here is a solution.

``````DateTime dateOfBirth = new DateTime(2000, 4, 18);
DateTime currentDate = DateTime.Now;

int ageInYears = 0;
int ageInMonths = 0;
int ageInDays = 0;

ageInDays = currentDate.Day - dateOfBirth.Day;
ageInMonths = currentDate.Month - dateOfBirth.Month;
ageInYears = currentDate.Year - dateOfBirth.Year;

if (ageInDays < 0)
{
ageInDays += DateTime.DaysInMonth(currentDate.Year, currentDate.Month);
ageInMonths = ageInMonths--;

if (ageInMonths < 0)
{
ageInMonths += 12;
ageInYears--;
}
}

if (ageInMonths < 0)
{
ageInMonths += 12;
ageInYears--;
}

Console.WriteLine("{0}, {1}, {2}", ageInYears, ageInMonths, ageInDays);
``````

Just because I don't think the top answer is that clear:

``````public static int GetAgeByLoop(DateTime birthday)
{
var age = -1;

for (var date = birthday; date < DateTime.Today; date = date.AddYears(1))
{
age++;
}

return age;
}
``````

I would simply do this:

``````DateTime birthDay = new DateTime(1990, 05, 23);
DateTime age = DateTime.Now - birthDay;
``````

This way you can calculate the exact age of a person, down to the millisecond if you want.

I have no knowledge about `DateTime` but all I can do is this:

``````using System;

public class Program
{
public static int getAge(int month, int day, int year) {
DateTime today = DateTime.Today;
int currentDay = today.Day;
int currentYear = today.Year;
int currentMonth = today.Month;
int age = 0;
if (currentMonth < month) {
age -= 1;
} else if (currentMonth == month) {
if (currentDay < day) {
age -= 1;
}
}
currentYear -= year;
age += currentYear;
return age;
}
public static void Main()
{
int ageInYears = getAge(8, 10, 2007);
Console.WriteLine(ageInYears);
}
}
``````

A little confusing, but looking at the code more carefully, it will all make sense.

I have a customized method to calculate age, plus a bonus validation message just in case it helps:

``````public void GetAge(DateTime dob, DateTime now, out int years, out int months, out int days)
{
years = 0;
months = 0;
days = 0;

DateTime tmpdob = new DateTime(dob.Year, dob.Month, 1);
DateTime tmpnow = new DateTime(now.Year, now.Month, 1);

{
months++;
if (months > 12)
{
years++;
months = months - 12;
}
}

if (now.Day >= dob.Day)
days = days + now.Day - dob.Day;
else
{
months--;
if (months < 0)
{
years--;
months = months + 12;
}
days += DateTime.DaysInMonth(now.AddMonths(-1).Year, now.AddMonths(-1).Month) + now.Day - dob.Day;
}

if (DateTime.IsLeapYear(dob.Year) && dob.Month == 2 && dob.Day == 29 && now >= new DateTime(now.Year, 3, 1))
days++;

}

private string ValidateDate(DateTime dob) //This method will validate the date
{
int Years = 0; int Months = 0; int Days = 0;

GetAge(dob, DateTime.Now, out Years, out Months, out Days);

if (Years < 18)
message =  Years + " is too young. Please try again on your 18th birthday.";
else if (Years >= 65)
message = Years + " is too old. Date of Birth must not be 65 or older.";
else
return null; //Denotes validation passed
}
``````

Method call here and pass out datetime value (MM/dd/yyyy if server set to USA locale). Replace this with anything a messagebox or any container to display:

``````DateTime dob = DateTime.Parse("03/10/1982");

string message = ValidateDate(dob);

lbldatemessage.Visible = !StringIsNullOrWhitespace(message);
lbldatemessage.Text = message ?? ""; //Ternary if message is null then default to empty string
``````

Remember you can format the message any way you like.

This gives "more detail" to this question. Maybe this is what you're looking for

``````DateTime birth = new DateTime(1974, 8, 29);
DateTime today = DateTime.Now;
TimeSpan span = today - birth;
DateTime age = DateTime.MinValue + span;

// Make adjustment due to MinValue equalling 1/1/1
int years = age.Year - 1;
int months = age.Month - 1;
int days = age.Day - 1;

// Print out not only how many years old they are but give months and days as well
Console.Write("{0} years, {1} months, {2} days", years, months, days);
``````

I use this:

``````public static class DateTimeExtensions
{
public static int Age(this DateTime birthDate)
{
return Age(birthDate, DateTime.Now);
}

public static int Age(this DateTime birthDate, DateTime offsetDate)
{
int result=0;
result = offsetDate.Year - birthDate.Year;

if (offsetDate.DayOfYear < birthDate.DayOfYear)
{
result--;
}

return result;
}
}
``````

This is one of the most accurate answers that is able to resolve the birthday of 29th of Feb compared to any year of 28th Feb.

``````public int GetAge(DateTime birthDate)
{
int age = DateTime.Now.Year - birthDate.Year;

if (birthDate.DayOfYear > DateTime.Now.DayOfYear)
age--;

return age;
}

``````

``````        DateTime dob = new DateTime(1991, 3, 4);
DateTime now = DateTime.Now;
int dobDay = dob.Day, dobMonth = dob.Month;
int add = -1;
if (dobMonth < now.Month)
{
}
else if (dobMonth == now.Month)
{
if(dobDay <= now.Day)
{
}
else
{
}
}
else
{
}
int age = now.Year - dob.Year + add;
``````

Here's a DateTime extender that adds the age calculation to the DateTime object.

``````public static class AgeExtender
{
public static int GetAge(this DateTime dt)
{
int d = int.Parse(dt.ToString("yyyyMMdd"));
int t = int.Parse(DateTime.Today.ToString("yyyyMMdd"));
return (t-d)/10000;
}
}
``````

The following approach (extract from Time Period Library for .NET class DateDiff) considers the calendar of the culture info:

``````// ----------------------------------------------------------------------
private static int YearDiff( DateTime date1, DateTime date2 )
{
return YearDiff( date1, date2, DateTimeFormatInfo.CurrentInfo.Calendar );
} // YearDiff

// ----------------------------------------------------------------------
private static int YearDiff( DateTime date1, DateTime date2, Calendar calendar )
{
if ( date1.Equals( date2 ) )
{
return 0;
}

int year1 = calendar.GetYear( date1 );
int month1 = calendar.GetMonth( date1 );
int year2 = calendar.GetYear( date2 );
int month2 = calendar.GetMonth( date2 );

// find the the day to compare
int compareDay = date2.Day;
int compareDaysPerMonth = calendar.GetDaysInMonth( year1, month1 );
if ( compareDay > compareDaysPerMonth )
{
compareDay = compareDaysPerMonth;
}

// build the compare date
DateTime compareDate = new DateTime( year1, month2, compareDay,
date2.Hour, date2.Minute, date2.Second, date2.Millisecond );
if ( date2 > date1 )
{
if ( compareDate < date1 )
{
compareDate = compareDate.AddYears( 1 );
}
}
else
{
if ( compareDate > date1 )
{
compareDate = compareDate.AddYears( -1 );
}
}
return year2 - calendar.GetYear( compareDate );
} // YearDiff
``````

Usage:

``````// ----------------------------------------------------------------------
public void CalculateAgeSamples()
{
PrintAge( new DateTime( 2000, 02, 29 ), new DateTime( 2009, 02, 28 ) );
// > Birthdate=29.02.2000, Age at 28.02.2009 is 8 years
PrintAge( new DateTime( 2000, 02, 29 ), new DateTime( 2012, 02, 28 ) );
// > Birthdate=29.02.2000, Age at 28.02.2012 is 11 years
} // CalculateAgeSamples

// ----------------------------------------------------------------------
public void PrintAge( DateTime birthDate, DateTime moment )
{
Console.WriteLine( "Birthdate={0:d}, Age at {1:d} is {2} years", birthDate, moment, YearDiff( birthDate, moment ) );
} // PrintAge
``````

To calculate how many years old a person is,

``````DateTime dateOfBirth;

int ageInYears = DateTime.Now.Year - dateOfBirth.Year;

if (dateOfBirth > today.AddYears(-ageInYears )) ageInYears --;
``````

My suggestion

``````int age = (int) ((DateTime.Now - bday).TotalDays/365.242199);
``````

That seems to have the year changing on the right date. (I spot tested up to age 107.)

``````DateTime dateOfBirth = Convert.ToDateTime("01/16/1990");
var age = ((DateTime.Now - dateOfBirth).Days) / 365;
``````

Simple Code

`````` var birthYear=1993;
var age = DateTime.Now.AddYears(-birthYear).Year;
``````

Check this out:

``````TimeSpan ts = DateTime.Now.Subtract(Birthdate);
age = (byte)(ts.TotalDays / 365.25);
``````

To calculate the age with nearest age:

``````var ts = DateTime.Now - new DateTime(1988, 3, 19);
var age = Math.Round(ts.Days / 365.0);
``````

Wow, I had to give my answer here... There are so many answers for such a simple question.

``````private int CalcularIdade(DateTime dtNascimento)
{
var nHoje = Convert.ToInt32(DateTime.Today.ToString("yyyyMMdd"));
var nAniversario = Convert.ToInt32(dtNascimento.ToString("yyyyMMdd"));

double diff = (nHoje - nAniversario) / 10000;

var ret = Convert.ToInt32(Math.Truncate(diff));

return ret;
}
``````

How come the MSDN help did not tell you that? It looks so obvious:

``````System.DateTime birthTime = AskTheUser(myUser); // :-)
System.DateTime now = System.DateTime.Now;
System.TimeSpan age = now - birthTime; // As simple as that
double ageInDays = age.TotalDays; // Will you convert to whatever you want yourself?
``````

I often count on my fingers. I need to look at a calendar to work out when things change. So that's what I'd do in my code:

``````int AgeNow(DateTime birthday)
{
return AgeAt(DateTime.Now, birthday);
}

int AgeAt(DateTime now, DateTime birthday)
{
return AgeAt(now, birthday, CultureInfo.CurrentCulture.Calendar);
}

int AgeAt(DateTime now, DateTime birthday, Calendar calendar)
{
// My age has increased on the morning of my
// birthday even though I was born in the evening.
now = now.Date;
birthday = birthday.Date;

var age = 0;
if (now <= birthday) return age; // I am zero now if I am to be born tomorrow.

while (calendar.AddYears(birthday, age + 1) <= now)
{
age++;
}
return age;
}
``````

Running this through in LINQPad gives this:

``````PASSED: someone born on 28 February 1964 is age 4 on 28 February 1968
PASSED: someone born on 29 February 1964 is age 3 on 28 February 1968
PASSED: someone born on 31 December 2016 is age 0 on 01 January 2017
``````

Code in LINQPad is here.

This is the version we use here. It works, and it's fairly simple. It's the same idea as Jeff's but I think it's a little clearer because it separates out the logic for subtracting one, so it's a little easier to understand.

``````public static int GetAge(this DateTime dateOfBirth, DateTime dateAsAt)
{
return dateAsAt.Year - dateOfBirth.Year - (dateOfBirth.DayOfYear < dateAsAt.DayOfYear ? 0 : 1);
}
``````

You could expand the ternary operator to make it even clearer, if you think that sort of thing is unclear.

Obviously this is done as an extension method on `DateTime`, but clearly you can grab that one line of code that does the work and put it anywhere. Here we have another overload of the Extension method that passes in `DateTime.Now`, just for completeness.