There are a few different ways to get the current date and time from the system using Python.
for example...
>>> import datetime
>>> datetime.datetime.now()
datetime.datetime(2013, 6, 10, 7, 59, 43, 458000)
>>>
This returns a datetime object
Your date field value can be retrived and compared to the now object.
The later date / time value will be greater
If your date fields are strings, you can turn them into datetaime objects using
datetime.strptime(date_string, format)
The python standard library has a list of the format tags.