Mind web2py offers all of this out of the box:
db.define_table('stuff',Field('name'), auth.signature)
auth.signature adds created_on, created_by, modified_on, modified_by and is_active. These fields can be used by auth for full auditing if enabled by
auth.enable_record_versioning(db)
Anyway, if you want to do this at lower level:
Field('mDate',default=f,update=g)
where f is a value or a a function that returns the value to be used when the record is created. g is a value or a function that returns a value to be used when the record is updated.
I would strongly suggest using auth.signature instead.