You can decide according to the definition of 'Time of supply' as per details given below:-
13. Time of supply of services. -(1) The liability to pay tax on services shall arise at the time of supply, as determined in accordance with the provisions of this section.
(2) The time of supply of services shall be the earliest of the following dates, namely :-
(a) the date of issue of invoice by the supplier, if the invoice is issued within the period prescribed under sub-section (2) of section 31 or the date of receipt of payment, whichever is earlier; or
(b) the date of provision of service, if the invoice is not issued within the period prescribed under sub-section (2) of section 31 or the date of receipt of payment, whichever is earlier; or
(c) the date on which the recipient shows the receipt of services in his books of account, in a case where the provisions of clause (a) or clause (b) do not apply :
Provided that where the supplier of taxable service receives an amount up to one thousand rupees in excess of the amount indicated in the tax invoice, the time of supply to the extent of such excess amount shall, at the option of the said supplier, be the date of issue of invoice relating to such excess amount.
If you have issued invoice, liability arises in the following month. If no invoice is issued, then the time of supply is the date of receipt of payment.