Date of calculating interest amount on delayed payment

I had booked a flat in year 2012 in Chandigarh. The agreement entered for the same has the execution date of Feb 20, 2012. When me and my mother entered into the agreement with the builder on Feb 20, 2012, we had already made a payment of Rs. 11 lac. Today I received a letter of possession from the builder and asked me to pay the pending amount. When I checked the statement of account, the builder had mentioned an amount of Rs. 25000 as interest on late payment. My question is which date will be considered the start of the relationship, the execution date of the agreement or when we started communicating with the builder? When we entered into the agreement the payment of Rs. 11lac was already made and they have charged interest on those installments.