explanation with an example

from datetime import datetime
fmt = '%Y-%m-%d'
from_date = '2013-09-01'     -> Take your date field1
to_date = '2013-09-31'       -> Take your date field2
d1 = datetime.strptime(from_date, fmt)
d2 = datetime.strptime(to_date, fmt)
daysDiff = str((d2-d1).days)          -> will gives you 29 days
daysDiff = str((d2-d1).days + 1)      -> will gives you 30 days

Hope it helps you ........ Thanks