Just FYI, I recently had to roll my own recurrence library before I found yours. I was checking out yours to possibly replace the one I wrote. After looking at it, I found a few issues with calculating future dates based on a monthly recurrence.

If my start date is on the 31st, it'll skip all the months that don't have 31 days.

Input

2013-01-31
Monthly

Output

2013-01-31
2013-03-31
2013-05-31
2013-07-31
2013-08-31

Also, if my start date is the last day of its month, I would expect all monthly frequencies to also be on the last day of each subsequent month.

Input

2013-02-28
Monthly

Output

2013-03-28 - (and doesn't include the first date, like the first example did)
2013-04-28
2013-05-28
2013-06-28
2013-07-28