Wyx dictionary

.FUNCTION....: M - Month Date Arithmetic
.CATEGORY....: functions
.DISCUSSION..:
Function name is "m" for "month arithmetic".

Here is an example:

       Fdate /Fm /Nnumdays /Adate /Iformat /Oformat

This function adds /N months to /Adate, and produces a date in /Oformat format. It can be used to do monthly subtraction by making the number in the /N parameter a negative number.

.EXAMPLES....:

 Fdate /Fm /N1  /A03-15-1992 /Imm-dd-ccyy /Omm-dd-ccyy
         produces:   04-15-1992

 Fdate /Fm /N-1 /A03-30-1991 /Imm-dd-ccyy /Omm-dd-ccyy
         produces:   02-28-1991

 Fdate /Fm /N-1 /A03-30-1992 /Imm-dd-ccyy /Omm-dd-ccyy
         produces:   02-29-1992

.DISCUSSION..:

Fdate's MONTH DATE ARITHMETIC IS SMART!

Month arithmetic that is done just by subtracting a certain number of months can produce non-existent dates. For example, subtracting one month from March 30, 1991 (as in the second example) could produce a result of February 30, 1991, a date which cannot exist.

Fdate's month arithmetic is smarter than that. If Fdate finds that a simple month-arithmetic operation produces an invalid date, it subtracts the minimum number of days required to produce a valid date.

In the second example, it produces February 28, 1991, the last date in February, 1991.
In the third example, it produces February 29, 1992 because 1992 is a leap year.
Telling Fdate to add 12 months to February 29, 1992 produces a result of February 28, 1993, since 1993 is not a leap year.

Problems with overly-simple date arithmetic are not merely academic. Here is a passage from Peter G. Neumann's INSIDE RISKS column in COMMUNICATIONS OF THE ACM, June 1992 (Vol. 35, No. 6). The column title was "Leap-Year Problems":

Prime Computer's MAGSAV failed at midnight [on Feb 29, 1992]... G. M. Lack noted that MAGSAV probably failed on leap-day because it tried to increment the year by one to set a tape label expiration date, and the resulting nonexistent date of Feb 29, 1993 threw it for a loop.