Service tax rate in reverse charge mechanism?

Dear Members,

Please resolve that if reverse charge mechanim applied and service provider issued the invoice in the month of may and also service is provided in the month of may itself and also payment made to service provider in the month of may, then should service receiver pay the service tax at old rate i.e. 12.36% or new rate i.e.14%? 

And what if the service is provided in the month of may and invoice is issued by the service provider in the month of June also the payment made to service provider in the month of June?

please help soon.