Propel group by date from timestamp

Propel is a very popular orm.This orm use for query data from database.In a project i fell a problem in propel.Like in my mysql database customer table i have a field that is created_at.In this created_at filed i keep timestamp value.Timestamp means there is save datetime format value.Now i need to query form this customer table to get group by as date from created_at field.I mean that i try to use mysql DATE() function.Here i gave and example how to i solve this.