' WEEKDAY$(3.0)  Form Day of a Week Abbreviated Name       09/30/1992-02/03/2010
      ' ------------------------------------------------------------------------------
      ' Copyright (C) 1992-2010 by Vladimir Veytsel                      www.davar.net

      ' Type -------------------------------------------------------------------------

      '    Function

      ' Description ------------------------------------------------------------------

      '    WEEKDAY$ function returns the name of the day of a week (abbreviated
      '    to three characters) for the specified date.

      ' Parameter --------------------------------------------------------------------

      '    Spec_Date$  - Specified date in the form of MM-DD-YY or MM-DD-CCYY
      '                  (should be a valid date in Gregorian calendar)

      ' Value ------------------------------------------------------------------------

      '    Three character day of the week identification for the specified date.

      ' Notes ------------------------------------------------------------------------

      '  - Delimiters of parameter date fields are irrelevant and can be any
      '    symbols.  Date fields are extracted from fixed positions.

      '  - IF century part is not specified (total year length is 2 characters),
      '       THEN CURRENT century is assumed (two digit of century get appended
      '            before YY).

      '  - If specified date is EMPTY,
      '       then day of a week identification is returned for the CURRENT date.

      '  - IF specified date doesn't represent a proper Gregorian calendar date,
      '       THEN "***" is returned instead of day of a week identification
      '            as an eye-catcher for an error in date specification.

      '  - Day of a week is determined by the formula that was derived from the
      '    algorithm suggested by Christian Zeller in 1883 (Zeller's Congruence).

      '    D  - Day   within specified month
      '    M  - Month within specified year
      '    Y  - Year (full 4-digit including century)

      '    If (M<3)
      '       M=M+12
      '       Y=Y-1

      '    WDI=(D+INT((13*M-27)/5)+Y+INT(Y/4)-INT(Y/100)+INT(Y/400)) MOD 7

      '    WDI - Week Day Index for the given date (0-Sun, 1-Mon, ..., 6-Sat)

      '  - Formula works for dates after 09/14/1752 - when England and its
      '    colonies switched from Julian calendar to currently used Gregorian.

      ' Examples ---------------------------------------------------------------------

      '    WEEKDAY$(""          )="Fri"  - Today 03-08-2002
      '    WEEKDAY$("*1-01-1996")="***"
      '    WEEKDAY$("12-31-1996")="Tue"
      '    WEEKDAY$("01-01-1997")="Wed"
      '    WEEKDAY$("12-31-1997")="Wed"
      '    WEEKDAY$("01-01-1998")="Thu"
      '    WEEKDAY$("12-31-1998")="Thu"
      '    WEEKDAY$("01-01-1999")="Fri"
      '    WEEKDAY$("12-31-1999")="Fri"
      '    WEEKDAY$("01-01-2000")="Sat"
      '    WEEKDAY$("12-31-2000")="Sun"
      '    WEEKDAY$("01-01-2001")="Mon"
      '    WEEKDAY$("12-31-2001")="Mon"
      '    WEEKDAY$("01-01-2002")="Tue"
      '    WEEKDAY$("12-31-2002")="Tue"
      '    WEEKDAY$("01-01-2003")="Wed"
      '    WEEKDAY$("12-31-2003")="Wed"
      '    WEEKDAY$("01-01-2004")="Thu"
      '    WEEKDAY$("12-31-04"  )="Fri"

      ' External Function ------------------------------------------------------------

           #INCLUDE ONCE "VALDGRE"

      ' Start Function ---------------------------------------------------------------

           DEFINT A-Z  ' All defaulted variables are integer

           FUNCTION WEEKDAY$(Spec_Date$)

      ' Check Date Validity and Handle Default Empty Date (Current) ------------------

           IF (LEN(Spec_Date$)=0) THEN
              DT$=DATE$
           ELSEIF (VALDGRE%(Spec_Date$)) THEN
              DT$=Spec_Date$
           ELSE
              WEEKDAY$="***"
              EXIT FUNCTION
           END IF

      ' Form Date Working Variables --------------------------------------------------

           Month=VAL(LEFT$(DT$,2))
           Day  =VAL( MID$(DT$,4,2))

           IF (LEN(DT$)=8) THEN
              Year=VAL(MID$(DATE$,7,2)+RIGHT$(DT$,2))  ' Use current century
           ELSE
              Year=VAL(RIGHT$(DT$,4))
           END IF

      ' Form and Return Day of a Week Value to the Point of Invocation ---------------

           IF (Month<3) THEN
              Month=Month+12
              Year=Year-1
           END IF

           Day_Index=(Day+INT((13*Month-27)/5)+Year+INT(Year/4)-INT(Year/100)+ _
                      INT(Year/400)) MOD 7

           WEEKDAY$=MID$("SunMonTueWedThuFriSat",3*Day_Index+1,3)

      ' Finish Function --------------------------------------------------------------

           END FUNCTION
  

      ' WEEKDAY$(3.0)  Form Day of a Week Abbreviated Name       09/30/1992-02/03/2010
      ' ------------------------------------------------------------------------------

        #INCLUDE "WEEKDAY"

        FUNCTION PBMAIN

        PRINT "WEEKDAY$(3.0)  Form Day of a Week Abbreviated Name  ";DATE$;
        PRINT "  ";LEFT$(TIME$, 5)
        PRINT STRING$(69,"-")
        PRINT

        PRINT "WEEKDAY$(''          )='"; WEEKDAY$(""); "'  - Today "; DATE$

        PRINT "WEEKDAY$('*1-01-1996')='"; _
               WEEKDAY$("*1-01-1996"); "'  - Should be '***' (error)"
        PRINT "WEEKDAY$('12-31-1996')='"; _
               WEEKDAY$("12-31-1996"); "'  - Should be 'Tue'"
        PRINT "WEEKDAY$('01-01-1997')='"; _
               WEEKDAY$("01-01-1997"); "'  - Should be 'Wed'"
        PRINT "WEEKDAY$('12-31-1997')='"; _
               WEEKDAY$("12-31-1997"); "'  - Should be 'Wed'"
        PRINT "WEEKDAY$('01-01-1998')='"; _
               WEEKDAY$("01-01-1998"); "'  - Should be 'Thu'"
        PRINT "WEEKDAY$('12-31-1998')='"; _
               WEEKDAY$("12-31-1998"); "'  - Should be 'Thu'"
        PRINT "WEEKDAY$('01-01-1999')='"; _
               WEEKDAY$("01-01-1999"); "'  - Should be 'Fri'"
        PRINT "WEEKDAY$('12-31-1999')='"; _
               WEEKDAY$("12-31-1999"); "'  - Should be 'Fri'"
        PRINT "WEEKDAY$('01-01-2000')='"; _
               WEEKDAY$("01-01-2000"); "'  - Should be 'Sat'"
        PRINT "WEEKDAY$('12-31-2000')='"; _
               WEEKDAY$("12-31-2000"); "'  - Should be 'Sun'"
        PRINT "WEEKDAY$('01-01-2001')='"; _
               WEEKDAY$("01-01-2001"); "'  - Should be 'Mon'"
        PRINT "WEEKDAY$('12-31-2001')='"; _
               WEEKDAY$("12-31-2001"); "'  - Should be 'Mon'"
        PRINT "WEEKDAY$('01-01-2002')='"; _
               WEEKDAY$("01-01-2002"); "'  - Should be 'Tue'"
        PRINT "WEEKDAY$('12-31-2002')='"; _
               WEEKDAY$("12-31-2002"); "'  - Should be 'Tue'"
        PRINT "WEEKDAY$('01-01-2003')='"; _
               WEEKDAY$("01-01-2003"); "'  - Should be 'Wed'"
        PRINT "WEEKDAY$('12-31-2003')='"; _
               WEEKDAY$("12-31-2003"); "'  - Should be 'Wed'"
        PRINT "WEEKDAY$('01-01-2004')='"; _
               WEEKDAY$("01-01-2004"); "'  - Should be 'Thu'"
        PRINT "WEEKDAY$('12-31-04'  )='"; _
               WEEKDAY$("12-31-04"  ); "'  - Should be 'Fri'"

        END FUNCTION