Time of supply of services determines when service tax liability arises under Section 13 CGST and related IGST rules. Time of Supply of services fixes the date when GST liability arises. Section 13 prescribes distinct rules for forward charge, reverse charge, voucher based supplies, a residual provision and inclusion of interest, late fee or penalty in value. These rules govern intra state supplies and, via section 20 of the IGST Act, apply to inter state supplies subject to integrated tax. Determination of the operative time may require concurrent reference to invoice issuance rules in Section 31.