[web2py] date operation

[ceriox]

hi,
how i can increment a data field of x days?

example
i have
db.Prestiti.Data_richiesta=2010-06-03 16:13:14
db.Prestiti.Durata=30

i want generate
fineprestito=2010-07-03 16:13:14

i wanna add 30 days to a data field