[web2py] Date Types

[villas]

I notice that:
    form.vars.date_field == str    
    form.vars.time_field == datetime.time
    form.vars.datetime_field == datetime.datetime

Maybe it's just an anomally?  I seem to lose a lot of time getting date and 
time fields from one format to another.  I wonder whether there is a good 
resource anywhere which explains how to convert, add and subtract them etc?