I do it like this:
(Shortened the code a bit)
public struct Age
{
public readonly int Years;
public readonly int Months;
public readonly int Days;
}
public Age( int y, int m, int d ) : this()
{
Years = y;
Months = m;
Days = d;
}
public static Age CalculateAge ( DateTime birthDate, DateTime anotherDate )
{
if( startDate.Date > endDate.Date )
{
throw new ArgumentException ("startDate cannot be higher then endDate", "startDate");
}
int years = endDate.Year - startDate.Year;
int months = 0;
int days = 0;
// Check if the last year, was a full year.
if( endDate < startDate.AddYears (years) && years != 0 )
{
years--;
}
// Calculate the number of months.
startDate = startDate.AddYears (years);
if( startDate.Year == endDate.Year )
{
months = endDate.Month - startDate.Month;
}
else
{
months = ( 12 - startDate.Month ) + endDate.Month;
}
// Check if last month was a complete month.
if( endDate < startDate.AddMonths (months) && months != 0 )
{
months--;
}
// Calculate the number of days.
startDate = startDate.AddMonths (months);
days = ( endDate - startDate ).Days;
return new Age (years, months, days);
}
// Implement Equals, GetHashCode, etc... as well
// Overload equality and other operators, etc...
}