In respect of Service Tax:
In terms of Point of taxation rules "Construction services" are Continous supply of service as work carried out for more than 3 months. Therfefore, in terms of proviso to Rule 3(b) the activity would be liable to service tax when contract requires for payment. If payment is not made on due date then last date when payment is required to be made would be point of tax.
Conclusion:
You are require to make payment of service tax when you make payment as per contract and if you would not make payment in time then last date when payment is require to be made would be point of taxation.
Regards