Para 7.1 of AS-9 states that, Revenue from service transactions is usually recognised as the service is performed , either by the proportionate completion method or by the completed service contract method.
Thus at the time of raising the invoices you cannot take the entire amount as your service income rather you should recognise it only on the proportionate basis.
At the time of invoice booking
Customer A/c Dr 1,20,000
To Deferred Income A/c (Liability) 1,06,800
To Service tax payable A/c 13,200
Month End closure entry
Deferred Income A/c (Liability) Dr 8,900 (1,06,800/12)
To Service Income A/c Cr 8,900
At the time of ST payable
Service tax A/c Dr 13,200
To Bank A/c 13,200
At the time of receipt of cash
Bank A/c Dr 1,20,000
To Customer A/c 1,20,000