Hi Peddie,
Yes, you can do this with qRules. Out-of-box InfoPath can probably do it too but it requires using ~31 rows in your default values dialog and marking them using a counter. I can show you how to do that, but it would take 15 minutes. qRules Insert command takes a /row parameter that allows you to insert them and you can calculate the number of days in a month using the DateDiff command. qRules comes with an hour of support and there's a free trial on qdabra (dot) com under Tools.
Cheers,