The International Standard week numbering system is ISO-8601. It specifies the following rules

• Week 1 is defined as the first week of a Gregorian year to contain a Thursday
• All ISO weeks start on a Monday and the days are numbered 1=Monday, 2=Tuesday, 3= Wednesday, ... 7 = Sunday
• Any January dates prior to start of week 1 will be numbered as if they were last week of previous year
• Years normally have 52 weeks but it is possible to have 53 ISO Weeks
• ISO Week dates are normally written in the format yyyy-Www-d where yyyy=ISO Year, -W is "-W", - is "-" and d = day-of-week (1=Mon 7=Sun)
• You may encounter non-standard week number systems (especially in spreadsheets,) this page relates to ISO-8601 compliant dates

Public Shared Function Gregorian_To_IsoWeek(date_Greg) As ISO_Week
 Dim ISO As ISO_Week
 ISO.yyyy = ISOWeek.GetYear(date_Greg)
 ISO.ww = ISOWeek.GetWeekOfYear(date_Greg)
 ISO.d = CInt(date_Greg.dayOfweek)
 If ISO.d = 0 Then ISO.d = 7 ' Sunday=0 in .Net Sunday=7 in ISO-Week
 ISO.IsoWeekDate = ISO.yyyy.ToString("0000") & "-W" & ISO.ww.ToString("00") & "-" & ISO.d.ToString("0")
 Return ISO
End Function

Public Structure ISO_Week
 Dim yyyy As Integer
 Dim ww As Integer
 Dim d As Integer
 Dim IsoWeekDate As String
End Structure
  

 

Public Shared Function IsoWeek_To_Gregorian(ISO_yyyy As Integer, ISO_ww As Integer, ISO_d As Integer) As Date
 Dim date_Greg As Date = ISOWeek.ToDateTime(ISO_yyyy, ISO_ww, ISO_d)
 ' fix time as midday - this avoids rare but obscure issues with clock changes etc.
 Return New Date(date_Greg.Year, date_Greg.Month, date_Greg.Day, 12, 0, 0, 0, 0)
End Function
  

 

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