First day date in week based on particular date

This function identifies the first day of week based on a particular date. Example: Sending 01.01.2010 will return 28.12.2009. M | T | W | T | F | S | S 28  29  30  31   1    2    3 First date in same week as 01.01.2010 date, is 28.12.2009.

''' <summary>Returns first day date in week based on specified date.</summary>
Private Function FirstDateOfWeek(ByVal dayInWeek As Date) As Date


    ' Calculate difference between specified date days of week and first day of week
    Dim Days As Integer = dayInWeek.DayOfWeek - CurrentCulture.DateTimeFormat.FirstDayOfWeek


    ' Add day difference to number of days in week then devide sum with days in week, reminder is number of days we need to substract
    Days = (7 + Days) Mod 7


    ' Substract difference of days from specified date
    Return dayInWeek.AddDays(-Days)
End Function