Time of supply??

 one cement company books of accounts. They were entered an agreement to the government and obtained royalty differment. Agreement entered in pre- GST period. As per the agreement, dealer is liable for payment of royalty wef 01.06.2021. However, they undertook mining activities since 2015. They didn't even pay single penny to government being that they were under differment scheme..Now my query is, does the tax payer liable to pay RCM ?? What is time of supply ..2015 or 2021??