Get the correct week number of a given date

Question

I have Googled a lot and found a lot of solutions  but none of them give me the correct week number for the 2012-12-31  Even the example on MSDN  link  fails   2012-12-31 is Monday  therefore it should be Week 1  but every method I tried gives me 53  Here are some of the methods  that I have tried   From the MDSN Library   DateTimeFormatInfo dfi   DateTimeFormatInfo CurrentInfo  Calendar cal   dfi Calendar   return cal GetWeekOfYear date  dfi CalendarWeekRule  dfi FirstDayOfWeek     Solution 2   return new GregorianCalendar GregorianCalendarTypes Localized  GetWeekOfYear date  CalendarWeekRule FirstFourDayWeek  DayOfWeek Monday     Solution 3   CultureInfo ciCurr   CultureInfo CurrentCulture  int weekNum   ciCurr Calendar GetWeekOfYear dtPassed  CalendarWeekRule FirstFourDayWeek  DayOfWeek Monday   return weekNum    Update  The following method actually returns 1 when date is 2012-12-31  In other words  my problem was that my methods were not following the ISO-8601 standard      This presumes that weeks start with Monday     Week 1 is the 1st week of the year with a Thursday in it  public static int GetIso8601WeekOfYear DateTime time           Seriously cheat   If its Monday  Tuesday or Wednesday  then it ll         be the same week  as whatever Thursday  Friday or Saturday are         and we always get those right     DayOfWeek day   CultureInfo InvariantCulture Calendar GetDayOfWeek time       if  day  gt   DayOfWeek Monday  amp  amp  day  lt   DayOfWeek Wednesday                time   time AddDays 3                 Return the week of our adjusted day     return CultureInfo InvariantCulture Calendar GetWeekOfYear time  CalendarWeekRule FirstFourDayWeek  DayOfWeek Monday

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Since there doesn t seem to be a  Net-culture that yields the correct ISO-8601 week number  I d rather bypass the built-in week determination altogether  and do the calculation manually  instead of attempting to correct a partially correct result    What I ended up with is the following extension method        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        First of all    int date DayOfWeek   6    7  determines the weekday number  0 monday  6 sunday   date AddDays -  int date DayOfWeek   6    7  determines the date of the monday preceiding the requested week number   Three days later is the target thursday  which determines what year the week is in   If you divide the  zero based  day-number within the year by seven  round down   you get the  zero based  week number in the year    In c   integer calculation results are round down implicitly

User · Answer

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  The type has the following signature  which should cover most ISO week needs  namespace System Globalization       public static class ISOWeek               public static int GetWeekOfYear DateTime date           public static int GetWeeksInYear int year           public static int GetYear DateTime date           public static DateTime GetYearEnd int year           public static DateTime GetYearStart int year           public static DateTime ToDateTime int year  int week  DayOfWeek dayOfWeek            You can find the source code here  UPDATE  These APIs have also been included in the 2 1 version of  NET Standard

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var cultureInfo   CultureInfo CurrentCulture  var calendar   cultureInfo Calendar   var calendarWeekRule   cultureInfo DateTimeFormat CalendarWeekRule  var firstDayOfWeek   cultureInfo DateTimeFormat FirstDayOfWeek  var lastDayOfWeek   cultureInfo LCID    1033   En-us                       DayOfWeek Saturday                       DayOfWeek Sunday   var lastDayOfYear   new DateTime date Year  12  31    var weekNumber   calendar GetWeekOfYear date  calendarWeekRule  firstDayOfWeek       Check if this is the last week in the year and it doesn t occupy the whole week return weekNumber    53  amp  amp  lastDayOfYear DayOfWeek    lastDayOfWeek           1            weekNumber    It works well both for US and Russian cultures  ISO 8601 also will be correct   cause Russian week starts at Monday

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DateTimeFormatInfo dfi   DateTimeFormatInfo CurrentInfo    DateTime date1   new DateTime 2011  1  1     Calendar cal   dfi Calendar     Console WriteLine   0 d   Week  1    2     date1                       cal GetWeekOfYear date1  dfi CalendarWeekRule                                         dfi FirstDayOfWeek                       cal ToString   Substring cal ToString   LastIndexOf        1

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Based on il guru s answer  I created this version for my own needs that also returns the year component            lt summary gt          This presumes that weeks start with Monday          Week 1 is the 1st week of the year with a Thursday in it           lt  summary gt           lt param name  time  gt The date to calculate the weeknumber for  lt  param gt           lt returns gt The year and weeknumber lt  returns gt           lt remarks gt          Based on Stack Overflow Answer  https   stackoverflow com a 11155102          lt  remarks gt      public static  short year  byte week  GetIso8601WeekOfYear DateTime time                   Seriously cheat   If its Monday  Tuesday or Wednesday  then it ll            be the same week  as whatever Thursday  Friday or Saturday are             and we always get those right         DayOfWeek day   CultureInfo InvariantCulture Calendar GetDayOfWeek time           if  day  gt   DayOfWeek Monday  amp  amp  day  lt   DayOfWeek Wednesday                        time   time AddDays 3                        Return the week of our adjusted day         var week    byte CultureInfo InvariantCulture Calendar GetWeekOfYear time  CalendarWeekRule FirstFourDayWeek  DayOfWeek Monday           return   short  week  gt   52  amp  time Month    1   time Year - 1   time Year   week

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This is the way   public int GetWeekNumber         CultureInfo ciCurr   CultureInfo CurrentCulture      int weekNum   ciCurr Calendar GetWeekOfYear DateTime Now  CalendarWeekRule FirstFourDayWeek  DayOfWeek Monday       return weekNum      Most important for is the CalendarWeekRule parameter   See here  https   msdn microsoft com query dev14 query appId Dev14IDEF1 amp l IT-IT amp k k System Globalization CalendarWeekRule  k TargetFrameworkMoniker- NETFramework

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If you don t have  NET 5 0  extend the DateTime class to include week number  public static class Extension       public static int Week this DateTime date            var day    int CultureInfo CurrentCulture Calendar GetDayOfWeek date           return CultureInfo CurrentCulture Calendar GetWeekOfYear date AddDays 4 -  day    0   7   day    CalendarWeekRule FirstFourDayWeek  DayOfWeek Monday

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These two methods will help  assumming our week starts on Monday       lt summary gt          Returns the weekId          lt  summary gt           lt param name  DateTimeReference  gt  lt  param gt           lt returns gt Returns the current week id lt  returns gt      public static DateTime GetDateFromWeek int WeekReference                  365 leap         int DaysOffset   0          if  WeekReference  gt  1                        DaysOffset   7              WeekReference   WeekReference - 1                    DateTime DT   new DateTime DateTime Now Year  1  1           int CurrentYear   DT Year          DateTime SelectedDateTime   DateTime MinValue           while  CurrentYear    DT Year                        int TheWeek   WeekReportData GetWeekId DT               if  TheWeek    WeekReference                                SelectedDateTime   DT                  break                            DT   DT AddDays 1 0D                      if  SelectedDateTime    DateTime MinValue                        throw new Exception  Please check week                       return SelectedDateTime AddDays DaysOffset              lt summary gt          Returns the weekId          lt  summary gt           lt param name  DateTimeReference  gt  lt  param gt           lt returns gt Returns the current week id lt  returns gt      public static int GetWeekId DateTime DateTimeReference                CultureInfo ciCurr   CultureInfo InvariantCulture          int weekNum   ciCurr Calendar GetWeekOfYear DateTimeReference          CalendarWeekRule FirstFullWeek  DayOfWeek Monday           return weekNum

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Here is an extension version and nullable version of il guru s answer  Extension  public static int GetIso8601WeekOfYear this DateTime time        var day   CultureInfo InvariantCulture Calendar GetDayOfWeek time       if  day  gt   DayOfWeek Monday  amp  amp  day  lt   DayOfWeek Wednesday                time   time AddDays 3              return CultureInfo InvariantCulture Calendar GetWeekOfYear time  CalendarWeekRule FirstFourDayWeek  DayOfWeek Monday      Nullable  public static int  GetIso8601WeekOfYear this DateTime  time        return time  GetIso8601WeekOfYear       Usages  new DateTime 2019  03  15  GetIso8601WeekOfYear      returns 11   DateTime   new DateTime 2019  03  15   GetIso8601WeekOfYear      returns 11   DateTime   null  GetIso8601WeekOfYear      returns null

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In PowerShell 7 x y  You will need both of the following codelines  if you have the need for the matching WeekYear   System Globalization ISOWeek   GetWeekOfYear  get-date    System Globalization ISOWeek   GetYear  get-date

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In  NET 3 0 and later you can use the ISOWeek GetWeekOfDate-Method   Note that the year in the year   week number format might differ from the year of the DateTime because of weeks that cross the year boundary

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As noted in this MSDN page there is a slight difference between ISO8601 week and  Net week numbering   You can refer to this article in MSDN Blog for a better explanation   ISO 8601 Week of Year format in Microsoft  Net    Simply put   Net allow weeks to be split across years while the ISO standard does not  In the article there is also a simple function to get the correct ISO 8601 week number for the last week of the year   Update The following method actually returns 1 for 2012-12-31 which is correct in ISO 8601  e g  Germany       This presumes that weeks start with Monday     Week 1 is the 1st week of the year with a Thursday in it  public static int GetIso8601WeekOfYear DateTime time           Seriously cheat   If its Monday  Tuesday or Wednesday  then it ll         be the same week  as whatever Thursday  Friday or Saturday are         and we always get those right     DayOfWeek day   CultureInfo InvariantCulture Calendar GetDayOfWeek time       if  day  gt   DayOfWeek Monday  amp  amp  day  lt   DayOfWeek Wednesday                time   time AddDays 3                 Return the week of our adjusted day     return CultureInfo InvariantCulture Calendar GetWeekOfYear time  CalendarWeekRule FirstFourDayWeek  DayOfWeek Monday

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There can be more than 52 weeks in a year  Each year has 52 full weeks   1 or  2  leap year  days extra  They make up for a 53th week    52 weeks   7days   364 days    So for each year you have at least one an extra day  Two for leap years  Are these extra days counted as separate weeks of their own   How many weeks there are really depends on the starting day of your week  Let s consider this for 2012    US  Sunday -  Saturday   52 weeks   one short 2 day week for 2012-12-30  amp  2012-12-31  This results in a total of 53 weeks  Last two days of this year  Sunday   Monday  make up their own short week    Check your current Culture s settings to see what it uses as the first day of the week   As you see it s normal to get 53 as a result    Europe  Monday -  Sunday   January 2dn  2012-1-2  is the first monday  so this is the first day of the first week  Ask the week number for the 1st of January and you ll get back 52 as it is considered part of 2011 last s week    It s even possible to have a 54th week  Happens every 28 years when the 1st of January and the 31st of December are treated as separate weeks  It must be a leap year too    For example  the year 2000 had 54 weeks  January 1st  sat  was the first one week day  and 31st December  sun  was the second one week day   var d   new DateTime 2012  12  31   CultureInfo cul   CultureInfo CurrentCulture   var firstDayWeek   cul Calendar GetWeekOfYear      d      CalendarWeekRule FirstDay      DayOfWeek Monday    int weekNum   cul Calendar GetWeekOfYear      d      CalendarWeekRule FirstDay      DayOfWeek Monday    int year   weekNum    52  amp  amp  d Month    1   d Year - 1   d Year  Console WriteLine  Year   0  Week   1    year  weekNum     Prints out  Year  2012 Week  54  Change CalendarWeekRule in the above example to FirstFullWeek or FirstFourDayWeek and you ll get back 53  Let s keep the start day on Monday since we are dealing with Germany   So week 53 starts on monday 2012-12-31  lasts one day and then stops   53 is the correct answer  Change the Culture to germany if want to to try it   CultureInfo cul   CultureInfo GetCultureInfo  de-DE

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The easiest way to determine the week number ISO 8601 style using c  and the DateTime class   Ask this  the how-many-eth thursday of the year is the thursday of this week  The answer equals the wanted week number   var dayOfWeek    int moment DayOfWeek     Make monday the first day of the week if  --dayOfWeek  lt  0      dayOfWeek   6     The whole nr of weeks before this thursday plus one is the week number var weekNumber    moment AddDays 3 - dayOfWeek  DayOfYear - 1    7   1

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public int GetWeekNumber        CultureInfo ciCurr   CultureInfo CurrentCulture     int weekNum   ciCurr Calendar GetWeekOfYear DateTime Now      CalendarWeekRule FirstFourDayWeek  DayOfWeek Monday      return weekNum

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A year has 52 weeks and 1 day or 2 in case of a lap year  52 x 7   364   2012-12-31 would be week 53  a week that would only have 2 days because 2012 is a lap year

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The question is  How do you define if a week is in 2012 or in 2013  Your supposition  I guess  is that since 6 days of the week are in 2013  this week should be marked as the first week of 2013    Not sure if this is the right way to go   That week started on 2012  On monday 31th Dec   so it should be marked as the last week of 2012  therefore it should be the 53rd of 2012   The first week of 2013 should start on monday the 7th   Now  you can handle the particular case of edge weeks  first and last week of the year  using the day of week information  It all depends on your logic

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C  to Powershell port from code above from il guru   function GetWeekOfYear  datetime   inputDate        day    System Globalization CultureInfo   InvariantCulture Calendar GetDayOfWeek  inputDate     if    day -ge  System DayOfWeek   Monday  -and   day -le  System DayOfWeek   Wednesday               inputDate    inputDate AddDays 3             Return the week of our adjusted day     weekofYear    System Globalization CultureInfo   InvariantCulture Calendar GetWeekOfYear  inputDate   System Globalization CalendarWeekRule   FirstFourDayWeek   System DayOfWeek   Monday     return  weekofYear

[c#] Get the correct week number of a given date

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