From which date interest on payment of flat is calculated

Hi, I bought a flat in Oct. 2016 and agreement was done on 07/01/2017. After that builder provide me the first demand letter on 27/02/2017 for plinth,first & fourth slab collectively and my bank paid it by 27/4/2017. Now builder is asking interest for delayed days from 07/01/2017 to 27/4/2017. I asked them that you have provide me demand letter on 27/02/2017 then how can they calculate from 07/01/2017 ? They are saying that once agreement is done from that date we are liable to get amount from customer. I want to know what is the way of calculate the interest ? Because bank won't release disbursement until i provide them the demand letter and builder provide demand letter on 27/02/2017 then how can they calculate it from 07/01/2017 ? As per them the interest amount is 40k but if i calculate it from 27/02/2017 then its 17k. Please someone advise on this. Thanks Anil