7.2 Day of Week Implementations

1. The following QBASIC implementation as a subroutine is valid for all Gregorian Dates up to the word limits of the implementation.

   SUB week1 (dow AS INTEGER, y1 AS LONG, m1 AS INTEGER, d1 AS INTEGER)
   m% = m1
   y& = y1
   IF m% < 3 THEN
         y& = y& - 1
         m% = m% + 12
   END IF
   dow = (d1 + (153 * m% - 457) \ 5 + INT(365.25# * y&) - INT(y& * .01) + INT(y& * .0025) + 2) MOD 7
   IF dow<0 THEN
         dow=dow+7
   END IF
   END SUB

2. The following QBASIC implementation as a subroutine is valid for all Gregorian Dates from March 1 of the year 0 or greater. It uses integer arithmetic

   SUB week2 (dow AS INTEGER, y1 AS LONG, m1 AS INTEGER, d1 AS INTEGER)
   m% = m1
   y& = y1
   IF m% < 3 THEN
         y& = y& - 1
         m% = m% + 12
   END IF
   dow = (d1 + (153 * m% - 457) \ 5 + 365 * y& +y& \ 4 - y& \ 100 + y& \ 400 + 2) MOD 7
   END SUB

3. A typical Visual Basic subroutine

   Sub JDtoWeek(jd As Double)
   'INPUT: jd = Julian Day Number
   'OUTPUT: modifies the caption of label dow
   i% = Int(jd + 1.5) Mod 7
   If i% < 0 Then i% = i% + 7
   dow.Caption = weekname(i%)
   End Sub

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Last Modified: April 7, 2005