How to determine the time of supply for goods and services?

For goods, the time of supply is the date ownership of goods is transferred to purchaser. It is usually date of invoice provided it is not later than 21 days after delivery of goods. If it is more than 21 days, then delivery date is the time of supply.

For services, the time of supply is the date when a service is performed in accordance to the contract. It is usually upon completion of job and delivery of deliverables.

If payment is received earlier than either delivery of goods or date of issuance of invoice, then payment date is time of supply. However, this occurrence seldom happens.

The time of supply is important as it determines the taxable period in which a transaction falls into.

In construction industry, it is very common that contractor lodge a claim for work done say for May 2015 in the month of June 2015. And the client will only certify the claim say at the end of June or in early July.

Eg.
Work done 31 May 2015
Certification date 31 July 2015

What would be the time of supply in this case?

Section 11(2)(c) of the GST Act 2014: time of supply is the time when it becomes certain that taxable supply has taken place.

Per Customs' Guide on Construction Industry, tax invoice or certificate of workdone (21 days rule applies) will trigger the time of supply.