on March 31 won't take you to March 2 or 3 (now the result will
be the last day of February.)
In general, now stepping by months from the last days of the current
month A will take you to the very last day of the target month B if
B is shorter than A.
The previous version would just step to March 31 and rely on mktime(3)
to correct the date. Despite its simplicity, such way was counter-intuitive
to users and caused pain to shell script writers.
Noticed by: Igor Timkin <ivt at gamma dot ru>
Approved by: brian
MFC after: 2 weeks