I want to create a function that raise an error when the user enters a not valid date,so if we enter a date before the current date it will show an error, any ideas?

My class looks like:

```
class business_trip(orm.Model):
_columns = {'start_trip': fields.datetime('Trip Starts on:',translate=True, required=False),
        'start_business': fields.datetime('Business Starts on:',translate=True, required=False),
        'end_business': fields.datetime('Business Ends on:',translate=True, required=False),
        'end_trip': fields.datetime('Trip Ends on:',translate=True, required=False),

```