Gst in advance related.

As you are supply of service you have to show gst two times one the month you take the advanced date and the month date on which it completed or say you get full money as per the contract between two
gst rate will be 18%
suppose you get advance in 29th Nov of rs 30000
gst calculation will be
total invoice value 30000
basic value will be 30000minus gst tax
and my gst will be( 30000 multiplex 18) division by (100 plus18) which taxable for the month of Nov
now I have said or per contract my work will complete by Jan 10 and the party gives me rest amount again I calculated as per above and taxable on Jan month