You may want to look at the python-dateutil module from http://niemeyer.net/python-dateutil it makes it easy to do date manipulation like adding/subtracting days, weeks, months, etc.
The _class part just makes a handy datepicker widget appear in the UI. The important part is to use form.vars instead of request.vars because then you'll get a date object like date(2011,2,21) instead of a string '2011-02-21' and can therefore do easy date manipulation and use it with the DAL.
