Calculate date from week number

Question

Anyone know an easy way to get the date of the first day in the week  monday here in Europe   I know the year and the week number  I m going to do this in C

User · Answer

One of the biggest problems I found was to convert from weeks to dates, and then from dates to weeks.

The main problem is when trying to get the correct week year from a date that belongs to a week of the previous year. Luckily System.Globalization.ISOWeek.GetYear handles this.

Here is my solution:

public class WeekOfYear
{
    public static (int Year, int Week) DateToWeekOfYear(DateTime date) =>
        (ISOWeek.GetYear(date), ISOWeek.GetWeekOfYear(date));

    public static bool ValidYearAndWeek(int year, int week) =>
           year >= 1 && year <= 9999 && week >= 1 && week <= 53 // bounds of year/week
        && !(year <= 1 && week <= 1) && !(year >= 9999 && week >= 53); // bounds of DateTime

    public int Year { get; }
    public int Week { get; }
    public virtual DateTime StartOfWeek { get; protected set; }
    public virtual DateTime EndOfWeek { get; protected set; }

    public virtual IEnumerable<DateTime> DaysInWeek =>
        Enumerable.Range(1, 10).Select(i => StartOfWeek.AddDays(i));

    public WeekOfYear(int year, int week)
    {
        if (!ValidYearAndWeek(year, week))
            throw new ArgumentException($"DateTime can't represent {week} of year {year}.");

        Year = year;
        Week = week;
        StartOfWeek = ISOWeek.ToDateTime(year, week, DayOfWeek.Monday);
        EndOfWeek = ISOWeek.ToDateTime(year, week, DayOfWeek.Sunday).AddDays(1).AddTicks(-1);
    }

    public WeekOfYear((int Year, int Week) week) : this(week.Year, week.Week) { }
    public WeekOfYear(DateTime date) : this(DateToWeekOfYear(date)) { }
}

The second biggest problem was the preference for weeks starting on Sundays in the US.

The solution I cam up with subclasses WeekOfYear from above, and manages the offset of the in the constructor (which converts week to dates) and DateToWeekOfYear (which converts from date to week).

public class UsWeekOfYear : WeekOfYear
{
    public static new (int Year, int Week) DateToWeekOfYear(DateTime date)
    {
        // if date is a sunday, return the next week
        if (date.DayOfWeek == DayOfWeek.Sunday) date = date.AddDays(1);
        return WeekOfYear.DateToWeekOfYear(date);
    }

    public UsWeekOfYear(int year, int week) : base(year, week)
    {
        StartOfWeek = ISOWeek.ToDateTime(year, week, DayOfWeek.Monday).AddDays(-1);
        EndOfWeek = ISOWeek.ToDateTime(year, week, DayOfWeek.Sunday).AddTicks(-1);
    }

    public UsWeekOfYear((int Year, int Week) week) : this(week.Year, week.Week) { }
    public UsWeekOfYear(DateTime date) : this(DateToWeekOfYear(date)) { }
}

Here is some test code:

public static void Main(string[] args)
{
    Console.WriteLine("== Last Week / First Week");
    Log(new WeekOfYear(2020, 53));
    Log(new UsWeekOfYear(2020, 53));

    Log(new WeekOfYear(2021, 1));
    Log(new UsWeekOfYear(2021, 1));

    Console.WriteLine("\n== Year Crossover (iso)");
    var start = new DateTime(2020, 12, 26);
    var i = 0;
    Log(start.AddDays(i), new WeekOfYear(start.AddDays(i++))); // 2020-12-26 - Sat
    Log(start.AddDays(i), new WeekOfYear(start.AddDays(i++))); // 2020-12-27 - Sun
    Log(start.AddDays(i), new WeekOfYear(start.AddDays(i++))); // 2020-12-28 - Mon
    Log(start.AddDays(i), new WeekOfYear(start.AddDays(i++))); // 2020-12-29 - Tue
    Log(start.AddDays(i), new WeekOfYear(start.AddDays(i++))); // 2020-12-30 - Wed
    Log(start.AddDays(i), new WeekOfYear(start.AddDays(i++))); // 2020-12-30 - Thu
    Log(start.AddDays(i), new WeekOfYear(start.AddDays(i++))); // 2021-01-01 - Fri
    Log(start.AddDays(i), new WeekOfYear(start.AddDays(i++))); // 2021-01-02 - Sat
    Log(start.AddDays(i), new WeekOfYear(start.AddDays(i++))); // 2021-01-03 - Sun
    Log(start.AddDays(i), new WeekOfYear(start.AddDays(i++))); // 2021-01-04 - Mon
    Log(start.AddDays(i), new WeekOfYear(start.AddDays(i++))); // 2021-01-05 - Tue
    Log(start.AddDays(i), new WeekOfYear(start.AddDays(i++))); // 2021-01-06 - Wed

    Console.WriteLine("\n== Year Crossover (us)");
    i = 0;
    Log(start.AddDays(i), new UsWeekOfYear(start.AddDays(i++))); // 2020-12-26 - Sat
    Log(start.AddDays(i), new UsWeekOfYear(start.AddDays(i++))); // 2020-12-27 - Sun
    Log(start.AddDays(i), new UsWeekOfYear(start.AddDays(i++))); // 2020-12-28 - Mon
    Log(start.AddDays(i), new UsWeekOfYear(start.AddDays(i++))); // 2020-12-29 - Tue
    Log(start.AddDays(i), new UsWeekOfYear(start.AddDays(i++))); // 2020-12-30 - Wed
    Log(start.AddDays(i), new UsWeekOfYear(start.AddDays(i++))); // 2020-12-30 - Thu
    Log(start.AddDays(i), new UsWeekOfYear(start.AddDays(i++))); // 2021-01-01 - Fri
    Log(start.AddDays(i), new UsWeekOfYear(start.AddDays(i++))); // 2021-01-02 - Sat
    Log(start.AddDays(i), new UsWeekOfYear(start.AddDays(i++))); // 2021-01-03 - Sun
    Log(start.AddDays(i), new UsWeekOfYear(start.AddDays(i++))); // 2021-01-04 - Mon
    Log(start.AddDays(i), new UsWeekOfYear(start.AddDays(i++))); // 2021-01-05 - Tue
    Log(start.AddDays(i), new UsWeekOfYear(start.AddDays(i++))); // 2021-01-06 - Wed

    var x = new UsWeekOfYear(2020, 53) as WeekOfYear;
}

public static void Log(WeekOfYear week)
{
    Console.WriteLine($"{week} - {week.StartOfWeek:yyyy-MM-dd} ({week.StartOfWeek:ddd}) - {week.EndOfWeek:yyyy-MM-dd} ({week.EndOfWeek:ddd})");
}

public static void Log(DateTime date, WeekOfYear week)
{
    Console.WriteLine($"{date:yyyy-MM-dd (ddd)} - {week} - {week.StartOfWeek:yyyy-MM-dd (ddd)} - {week.EndOfWeek:yyyy-MM-dd (ddd)}");
}

User · Answer

The easiest way is probably to find the first Monday of the year  and then add the relevant number of weeks  Here s some sample code  It assumes a week number starting at 1  by the way   using System   class Test       static void Main                    Show the third Tuesday in 2009  Should be January 20th         Console WriteLine YearWeekDayToDateTime 2009  DayOfWeek Tuesday  3               static DateTime YearWeekDayToDateTime int year  DayOfWeek day  int week                DateTime startOfYear   new DateTime  year  1  1               The  7 and  7 stuff is to avoid negative numbers etc          int daysToFirstCorrectDay      int day -  int startOfYear DayOfWeek    7    7           return startOfYear AddDays 7    week-1    daysToFirstCorrectDay

User · Answer

I have written and tested the following code and is working perfectly fine for me  Please let me know if anyone face trouble with this  I have posted a question as well in order to get the best possible answer  Someone may find it useful   public static DateTime GetFirstDateOfWeekByWeekNumber int year  int weekNumber                        var date   new DateTime year  01  01               var firstDayOfYear   date DayOfWeek              var result   date AddDays weekNumber   7                if  firstDayOfYear    DayOfWeek Monday                  return result Date              if  firstDayOfYear    DayOfWeek Tuesday                  return result AddDays -1  Date              if  firstDayOfYear    DayOfWeek Wednesday                  return result AddDays -2  Date              if  firstDayOfYear    DayOfWeek Thursday                  return result AddDays -3  Date              if  firstDayOfYear    DayOfWeek Friday                  return result AddDays -4  Date              if  firstDayOfYear    DayOfWeek Saturday                  return result AddDays -5  Date              return result AddDays -6  Date

User · Answer

I improved a little on Thomas  solution with an override       public static DateTime FirstDateOfWeek int year  int weekOfYear              return Timer FirstDateOfWeekOfMonth year  1  weekOfYear              public static DateTime FirstDateOfWeekOfMonth int year  int month       int weekOfYear              DateTime dtFirstDayOfMonth   new DateTime year  month  1             I also commented out this part                 if  firstWeek  lt   1                  weekOfYear -  1                     Otherwise the date was preceding by one week    Thank you Thomas  great help

User · Answer

The free Time Period Library for  NET includes the ISO 8601 conform class Week      ---------------------------------------------------------------------- public static DateTime GetFirstDayOfWeek  int year  int weekOfYear       return new Week  year  weekOfYear   FirstDayStart       GetFirstDayOfWeek

User · Answer

I tried some codes above and some have small mistakes  when you try different years with different starting days of week you will see them  I took the code of Jon Skeet  fix it and it works  very simple code   Public Function YearWeekDayToDateTime ByVal year As Integer  ByVal weekDay As Integer  ByVal week As Integer  As DateTime      weekDay  day you want     Dim startOfYear As New DateTime year  1  1      Dim startOfYearFixDay As Integer      If startOfYear DayOfWeek  lt  gt  DayOfWeek Sunday Then         startOfYearFixDay   startOfYear DayOfWeek     Else         startOfYearFixDay   7     End If      Return startOfYear AddDays  7    week   - startOfYearFixDay   weekDay  End Function

User · Answer

Week 1 is defined as being the week that starts on a Monday and contains the first Thursday of the year

User · Answer

UPDATE   NET Core 3 0 and  NET Standard 2 1 has shipped with this type  Good news  A pull request adding System Globalization ISOWeek to  NET Core was just merged and is currently slated for the 3 0 release  Hopefully it will propagate to the other  NET platforms in a not-too-distant future  You should be able to use the ISOWeek ToDateTime int year  int week  DayOfWeek dayOfWeek  method to calculate this  You can find the source code here

User · Answer

According to ISO 8601 1988 that is used in Sweden the first week of the year is the first week that has at least four days within the new year   So if your week starts on a Monday the first Thursday any year is within the first week  You can DateAdd or DateDiff from that

User · Answer

Personally I d take advantage of the culture info to get the day of the week and loop down to the culture s first day of the week   I m not sure if I m explaining it properly  here s an example       public DateTime GetFirstDayOfWeek int year  int weekNumber                return GetFirstDayOfWeek year  weekNumber  Application CurrentCulture              public DateTime GetFirstDayOfWeek int year  int weekNumber          System Globalization CultureInfo culture                System Globalization Calendar calendar   culture Calendar          DateTime firstOfYear   new DateTime year  1  1  calendar           DateTime targetDay   calendar AddWeeks firstOfYear  weekNumber           DayOfWeek firstDayOfWeek   culture DateTimeFormat FirstDayOfWeek           while  targetDay DayOfWeek    firstDayOfWeek                        targetDay   targetDay AddDays -1                      return targetDay

User · Answer

Assuming the week number starts at 1  DateTime dt    new DateTime YearNumber  1  1  AddDays  WeekNumber - 1    7 -  WeekNumber    1   0   1    return dt AddDays - int dt DayOfWeek     This should give you the first day in any given week  I haven t done a lot of testing on it  but looks like it works  It s smaller solution than most other s I found on the web  so wanted to share

User · Answer

I have made a refined version of the proposed solution that is a simpler and parametrises the firstDayOfWeek   public static DateTime GetFirstDayOfWeek int year  int week  DayOfWeek firstDayOfWeek        return GetWeek1Day1 year  firstDayOfWeek  AddDays 7    week - 1       public static DateTime GetWeek1Day1 int year  DayOfWeek firstDayOfWeek        DateTime date   new DateTime year  1  1           Move towards firstDayOfWeek     date   date AddDays firstDayOfWeek - date DayOfWeek           Either 1 or 52 or 53     int weekOfYear   CultureInfo CurrentCulture Calendar GetWeekOfYear date  CalendarWeekRule FirstFullWeek  firstDayOfWeek           Move forwards 1 week if week is 52 or 53     date   date AddDays 7   System Math Sign weekOfYear - 1         return date

User · Answer

I like the solution provided by Henk Holterman  But to be a little more culture independent  you have to get the first day of the week for the current culture   it s not always monday     using System Globalization   static DateTime FirstDateOfWeek int year  int weekOfYear      DateTime jan1   new DateTime year  1  1      int daysOffset    int CultureInfo CurrentCulture DateTimeFormat FirstDayOfWeek -  int jan1 DayOfWeek     DateTime firstMonday   jan1 AddDays daysOffset      int firstWeek   CultureInfo CurrentCulture Calendar GetWeekOfYear jan1  CultureInfo CurrentCulture DateTimeFormat CalendarWeekRule  CultureInfo CurrentCulture DateTimeFormat FirstDayOfWeek      if  firstWeek  lt   1          weekOfYear -  1         return firstMonday AddDays weekOfYear   7

User · Answer

This one worked for me  it also have the advantage of expecting a cultureinfo as parameter to test the formula with different cultures  If empty  it gets the current culture info    valid values are like   it    en-us    fr       ando so on  The trick is to subtract the week number of the first day of the year  that may be 1 to indicate that the first day is within the first week  Hope this helps   Public Shared Function FirstDayOfWeek ByVal year As Integer  ByVal weekNumber As Integer  ByVal culture As String  As Date     Dim cInfo As System Globalization CultureInfo     If culture      Then         cInfo   System Globalization CultureInfo CurrentCulture     Else         cInfo   System Globalization CultureInfo CreateSpecificCulture culture      End If     Dim calendar As System Globalization Calendar   cInfo Calendar     Dim firstOfYear As DateTime   New DateTime year  1  1  calendar      Dim firstDayWeek As Integer   calendar GetWeekOfYear firstOfYear  cInfo DateTimeFormat CalendarWeekRule  cInfo DateTimeFormat FirstDayOfWeek      weekNumber -  firstDayWeek     Dim targetDay As DateTime   calendar AddWeeks firstOfYear  weekNumber      Dim fDayOfWeek As DayOfWeek   cInfo DateTimeFormat FirstDayOfWeek      While  targetDay DayOfWeek  lt  gt  fDayOfWeek          targetDay   targetDay AddDays -1      End While     Return targetDay End Function

User · Answer

I had issues with the solution by  HenkHolterman even with the fix by  RobinAndersson   Reading up on the ISO 8601 standard resolves the issue nicely  Use the first Thursday as the target and not Monday  The code below will work for Week 53 of 2009 as well   public static DateTime FirstDateOfWeekISO8601 int year  int weekOfYear        DateTime jan1   new DateTime year  1  1       int daysOffset   DayOfWeek Thursday - jan1 DayOfWeek          Use first Thursday in January to get first week of the year as        it will never be in Week 52 53     DateTime firstThursday   jan1 AddDays daysOffset       var cal   CultureInfo CurrentCulture Calendar      int firstWeek   cal GetWeekOfYear firstThursday  CalendarWeekRule FirstFourDayWeek  DayOfWeek Monday        var weekNum   weekOfYear         As we re adding days to a date in Week 1         we need to subtract 1 in order to get the right date for week  1     if  firstWeek    1                weekNum -  1                Using the first Thursday as starting week ensures that we are starting in the right year        then we add number of weeks multiplied with days     var result   firstThursday AddDays weekNum   7           Subtract 3 days from Thursday to get Monday  which is the first weekday in ISO8601     return result AddDays -3

User · Answer

To convert in both directions  see here  Wikipedia article on ISO week dates

User · Answer

The proposed solution is not complete - it only works for CalendarWeekRule FirstFullWeek   Other types of week rules do not work   This can be seen using this test case   foreach  CalendarWeekRule rule in Enum GetValues typeof CalendarWeekRule          for  int year   1900  year  lt  2000  year                  DateTime date   FirstDateOfWeek year  1  rule           Assert CultureInfo CurrentCulture Calendar GetWeekOfYear date  rule  DayOfWeek Monday     1           Assert CultureInfo CurrentCulture Calendar GetWeekOfYear date AddDays -1   rule  DayOfWeek Monday     1

User · Answer

this is my solution when we want to calculate a date given year  week number and day of the week   int Year   2014  int Week   48  int DayOfWeek   4   DateTime FecIni   new DateTime Year  1  1   FecIni   FecIni AddDays 7    Week - 1    if   int FecIni DayOfWeek  gt  DayOfWeek        while   int FecIni DayOfWeek    DayOfWeek  FecIni   FecIni AddDays -1     else       while   int FecIni DayOfWeek    DayOfWeek  FecIni   FecIni AddDays 1

User · Answer

I simplified the code Mikael Svensson provided which is correct for many countries in Europe   public static DateTime FirstDateOfWeekIso8601 int year  int week            var firstThursdayOfYear   new DateTime year  1  1           while  firstThursdayOfYear DayOfWeek    DayOfWeek Thursday                        firstThursdayOfYear   firstThursdayOfYear AddDays 1                      var startDateOfWeekOne   firstThursdayOfYear AddDays - DayOfWeek Thursday - DayOfWeek Monday             return startDateOfWeekOne AddDays 7    week - 1

User · Answer

I used one of the solutions but it gave me wrong results  simply because it counts Sunday as a first day of the week   I changed    var firstDay   new DateTime DateTime Now Year  1  1  AddDays  weekNumber - 1    7   var lastDay   firstDay AddDays 6     to    var lastDay   new DateTime DateTime Now Year  1  1  AddDays  weekNumber    7   var firstDay   lastDay AddDays -6     and now it is working as a charm

User · Answer

Currently  there is no C  class that correctly handles ISO 8601week numbers  Even though you can instantiate a culture  look for the closest thing and correct that  I think it is better to do the complete calculation yourself            lt summary gt          Converts a date to a week number          ISO 8601 week 1 is the week that contains the first Thursday that year           lt  summary gt      public static int ToIso8601Weeknumber this DateTime date                var thursday   date AddDays 3 - date DayOfWeek DayOffset             return  thursday DayOfYear - 1    7   1                  lt summary gt          Converts a week number to a date          Note  Week 1 of a year may start in the previous year          ISO 8601 week 1 is the week that contains the first Thursday that year  so         if December 28 is a Monday  December 31 is a Thursday          and week 1 starts January 4          If December 28 is a later day in the week  week 1 starts earlier          If December 28 is a Sunday  it is in the same week as Thursday January 1           lt  summary gt      public static DateTime FromIso8601Weeknumber int weekNumber  int  year   null  DayOfWeek day   DayOfWeek Monday                var dec28   new DateTime  year    DateTime Today Year  - 1  12  28           var monday   dec28 AddDays 7   weekNumber - dec28 DayOfWeek DayOffset             return monday AddDays day DayOffset                     lt summary gt          Iso8601 weeks start on Monday  This returns 0 for Monday           lt  summary gt      private static int DayOffset this DayOfWeek weekDay                return   int weekDay   6    7

User · Answer

Lightly changed Mikael Svenson code  I found the week of the first monday and appropriate change the week number    DateTime GetFirstWeekDay int year  int weekNum                Calendar calendar   CultureInfo CurrentCulture Calendar           DateTime jan1   new DateTime year  1  1            int daysOffset   DayOfWeek Monday - jan1 DayOfWeek          DateTime firstMonday   jan1 AddDays daysOffset           int firstMondayWeekNum   calendar GetWeekOfYear firstMonday  CalendarWeekRule FirstFourDayWeek  DayOfWeek Monday            DateTime firstWeekDay   firstMonday AddDays  weekNum-firstMondayWeekNum    7            return firstWeekDay

User · Answer

using Fluent DateTime http   fluentdatetime codeplex com           var year   2009          var firstDayOfYear   new DateTime year  1  1           var firstMonday   firstDayOfYear Next DayOfWeek Monday           var weeksDateTime   12 Weeks   Since firstMonday

User · Answer

Here is a method that is compatible with the week numbers that Google Analytics  and also the same numbering scheme we used internally at Intel  and which I m sure is also used in a lot of other contexts      Google Analytics does not follow ISO standards for date     It numbers week 1 starting on Jan  1  regardless what day of week it starts on     It treats Sunday as the first day of the week     The first and last weeks of a year are usually not complete weeks  public static DateTime GetStartDateTimeFromWeekNumberInYear int year  uint weekOfYear      if  weekOfYear    0    weekOfYear  gt  54  throw new ArgumentException  Week number must be between 1 and 54   Yes  54    Year 2000 had Jan  1 on a Saturday plus 53 Sundays            January 1 -- first week    DateTime firstDayInWeek   new DateTime year  1  1     if  weekOfYear    1  return firstDayInWeek        Get second week  starting on the following Sunday          do         firstDayInWeek   firstDayInWeek AddDays 1       while  firstDayInWeek DayOfWeek    DayOfWeek Sunday      if  weekOfYear    2  return firstDayInWeek        Now get the Sunday of whichever week we re looking for    return firstDayInWeek AddDays  weekOfYear - 2  7

[c#] Calculate date from week number

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