GST AGAINST ADVANCES

any difference between advance and actual bill? what's the date of billing

As per Notification 40 & 66/2017 CT the taxpayer is not required to pay GST on advances against Supply of GOODS. (Except Composition dealer)
Effective from 13/10/2017 for taxpayer turnover below 1.5 Crore & 15/11/2017 for Taxpayer whose turnover above 1.5 Crore.

Hence if you have reced Adance against supply goods then No GST liability leviable at the time of Advance reced.

If it is supply of goods - GST is payable on invoices itself - 

If supply of services - Interest needs to be paid for the period from date of GST payable (due date for payment for advance received) til the actual date of payment..