How do I compute the last day of the month?

The month has always been the most frustrating component of date arithmetic. Is it a 30-
day month? A 31-day month? Is it February? In a leap year? The code below calculates the 
last day of the month in which any date (DateIn) occurs. The logic is so simple that it's 
easy to overlook: The first day of the following month is always the 1st, and the day 
before that is the last day of the month. This code adds a month to the date, changes the 
day to the first, then subtracts a single day. Voil? ... DateEnd will be the last day of 
the month.

      **********************************************************************
      *
      *     LastDay   Determines last day of month for a date
      *
      **********************************************************************

     D DateIn          S               D

     D                 DS
     D DateEnd                         D   DATFMT(*ISO)
     D  DateDay                       2    OVERLAY(DateEnd:9)

     C     DateIn        ADDDUR    1:*M          DateEnd
     C                   EVAL      DateDay = '01'
     C                   SUBDUR    1:*D          DateEnd

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