Interest rate for delayed GST payments: differentiated rates and temporary nil interest windows affect compliance timing. Fixes annual rates of interest under UTGST by reference to specified CGST and UTGST provisions, prescribing 18% for interest under section 21 read with subsection (1) of section 50, 6% where section 21 is read with subsection (12) of section 54 and with section 56, and 9% for the proviso to section 56; includes turnover based, time limited nil and reduced interest windows and tabulated transitional concessions for defaulting registered persons, and notes subsequent insertions and substitutions amending periods and classes of taxpayers.