Cron parser and validation in python

The Croniter package seems like it may get what you need. Example from the docs:

>>> from croniter import croniter
>>> from datetime import datetime
>>> base = datetime(2010, 1, 25, 4, 46)
>>> iter = croniter('*/5 * * * *', base)  # every 5 minites
>>> print iter.get_next(datetime)   # 2010-01-25 04:50:00
>>> print iter.get_next(datetime)   # 2010-01-25 04:55:00
>>> print iter.get_next(datetime)   # 2010-01-25 05:00:00
>>>
>>> iter = croniter('2 4 * * mon,fri', base)  # 04:02 on every Monday and Friday
>>> print iter.get_next(datetime)   # 2010-01-26 04:02:00
>>> print iter.get_next(datetime)   # 2010-01-30 04:02:00
>>> print iter.get_next(datetime)   # 2010-02-02 04:02:00

Per the code, it also appears to do validation on the entered format. Likely that you came across this already, but just in case :)

Since the accepted answer is quite old, the same library has a croniter.is_valid() method now. From docs:

>>> croniter.is_valid('0 0 1 * *')  # True
>>> croniter.is_valid('0 wrong_value 1 * *')  # False