Gratuity calculation for private organisations

Dear sir, 

 gratuity calculation for private registered Society falling under the ambit of payment of gratuity act 1972. 

My specific query--is there any ceiling limit on the number of years of service to be taken for calculating gratuity? Like ceiling of 33 years even if an employee retires after service of 37 years? Or the full tenure of 37 years is taken?

Regards 

Thank you