advance tax on services and goods

 Works Contact- Supply of Service or Supply of Goods?

A simpler treatment has been introduced for Works Contract under GST. Schedule II clearly states that Works Contract amounts to supply of services, hence the confusion whether it will be categorised as supply of service or goods has been done away with. A single rate has been fixed for services provided under Works contract and the entire amount shall be taxed at this rate without any bifurcation between goods and services.

As per the CGST Notification Advance Received for goods is Exempt from GST

But in your Scenario, It comes under Services So you need to pay GST on Advance Received in Dec'17 by 20th Jan'18...