You need the datetime module from the standard library. Load the date string via strptime(), use timedelta to add a day, then use strftime() to dump the date back to a string:
>>> from datetime import datetime, timedelta
>>> s = '2004/03/30'
>>> date = datetime.strptime(s, "%Y/%m/%d")
>>> modified_date = date + timedelta(days=1)
>>> datetime.strftime(modified_date, "%Y/%m/%d")
'2004/03/31'
