Billing date should not be before service start date

Dear All ,

Im New to SAP , Request your valid inputs for the below scenario,

Business Scenario : Its a Service order ( Service order - Billing )

Sale order Created Date : 10-02-2013

               Document Date: 06-02-2013

                  Billing Date : 31-01-2013

We dont want the user to create a billing document before the service start date (i.e) 06.02.2013 ,as mentioned above it has been billed on 31.01.2013,

please suggest ,how to block the billing date ,which is less then the service start date,

Regards

Jeeva