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RPGIII Get Day of Week Routine (Zeller Conguence)
Published by: Bob Cozzi on 22 Dec 2011 view comments(2)
This is the RPGIII source code to calculate the day of the week, using Zeller's Congruence formula:
*****************************************************************
** Zeller's Congruence in RPGIII by Bob Cozzi, July 2006
** (c) 2006 by Bob Cozzi. All rights reserved.
***********************************************************
** Temp/work fields for calculation
ITEMPS IDS
I B 1 40DAYOWK
I B 5 80TEMP1
I B 9 120TEMP2
**
** Put the date into this Data Structure
IZDATE IDS
I 1 40NYEAR
I 5 60NMONTH
I 7 80NDAY
** Demo date is 23-July-1958 (should be a Wednesday)
C Z-ADD1958 NYEAR
C Z-ADD07 NMONTH
C Z-ADD23 NDAY
C EXSR ZDAY
C MOVE *ON *INLR
***********************************************************
** This subroutine calculates the day of the week using
** Zeller's congruence.
** --Converted to RPGIII by Bob Cozzi, July 2006
** (c) 2006 by Bob Cozzi. All rights reserved.
** Permission to use for any purpose is hereby granted
** provided credit to the copyright holder is included.
**
** Input fields:
** zDate Date in YYYYMMDD format
** nDay Day of month
** nYear Year
** nMonth Month
***********************************************************
CSR ZDAY BEGSR
** If the month Jan or Feb, then adjust it to fit the algorythm.
** Also subtract 1 from the year in this situation.
C NMONTH IFLT 3
C ADD 12 NMONTH
C SUB 1 NYEAR
C ENDIF
**
** Perform Zeller's formula to calculate the day of the week.
C NMONTH MULT 13 TEMP1
C ADD 3 TEMP1
C DIV 5 TEMP1
C ADD NDAY TEMP1
C ADD NYEAR TEMP1
C NYEAR DIV 4 TEMP2
C ADD TEMP2 TEMP1
**
C NYEAR DIV 100 TEMP2
C SUB TEMP2 TEMP1
C NYEAR DIV 400 TEMP2
C ADD TEMP2 TEMP1
C DIV 7 TEMP1
C MVR DAYOWK
**
** At this point, we have the day of the week from Zeller.
** However, it is zero-based (Mon=0, Tues=1, Wed=3... etc.)
** This procedure is designed to return ones-based values
** as follows: (Sun=1, Mon=2, Tues=3... etc.)
**
** We need to convert the result from Zeller, as follows:
** If day is greater than 5, subtract 5 from the day.
** Otherwise, increase the day by 2.
** For example: Sun=6 so 6 - 5 = 1
** Mon=0 so 0 + 2 = 2
** Tue=1 so 1 + 2 = 3
** Wed=2 so 2 + 2 = 4
** Thu=3 so 3 + 2 = 5
** Fri=4 so 4 + 2 = 6
** Sat=5 so 5 + 2 = 7
**
** If you prefer 0=Monday based return value, simply delete
** the code upto but NOT including the RETURN opcode.
C DAYOWK IFGT 5
C SUB 5 DAYOWK
C ELSE
C ADD 2 DAYOWK
C ENDIF
CSR ENDZDY ENDSR
