Reverse Charge Mechanism

in case of RCM on service for example legal services book the bill as usual.
1) if you make payment of legal expense within 60 days then create GST liability on legal service amount on the day when you make payment of legal service and pay GST within 20th day of the month following the month in which GST liability is created
2) if payment is not done within 60 days then create GST liability on 61st day and make payment on 20th day of the month following the month in which GST liability is created.