#17 Date calculations

[Eelco Vriezekolk]

All dates calculations can be give odd results by manipulating months
only: a due date of 30-Nov-2001 with a grace period of 3 months would
cause a cutoff after 30-Feb-2002. It may mistakenly give people an
additional day grace if their cutoff date was 28-Feb.

Use a Perl module for data calculations? Use of non-standard modules always makes software more difficult to install / port.