Can builder give possession before receiving the completion certi

Dear Sir, I have purchased an apartment in Noida ( Aditya Celebrity Homes , Sector 76 Noida) which was under construction at the time of purchasing. The completion certificate given to the builder by the Noida Authorities in September 2015, where as the builder started giving possession much earlier even without the completion certificate. I have not taken possession at that time due to absence of completion certificate and I had informed the same to the builder. Some people taken the possession and now the builder is charging the maintenance charges from April 2015. I understand that as per rule, builder cannot give possession before the completion charges so how can he charge the maintenance from April 2015 ( where as the completion certificate was received in September 2015. He is insisting the same date even to the people who are not yet given possession in spite the full and final payment is given to the builder. I will appreciate if you could clarify if it is legal for the builder to charge the maintenance charges even when the completion certificate was not available. Also advice me if the completion certificate is received after the scheduled date of possession ( which was promised in the contract), a buyer can ask the builder to pay the delay charges which was again promised in the deal document.