1. There is a difference of only 2 months between end date of 18m as per your and society's calculations
2. Ideally your calculations would prevail. Its based on logic. No specific regulation is needed for that
3. 18m has to be computed from date of handover of possession by builder to flat buyer
4. But there may be several flats in your society. And different buyers may have been granted possession on different dates depending on their making of full and final payment to builder
5. Many a times builder receives OC but does not handover possession to buyer as latter has to make balance payments to builder
6. So different buyers would be given possession on different dates
7. The MC of society has skipped the exercise of computing maintenance for individual flats
8. There is also no point taking this legally as the legal expenses will be far more than the excess money which you are required to be paid by society, unless you want to pursue the matter on a matter of principle and fairness
9. So i suggest you make the excess payment under protest under cover of a letter wherein you can state your objection and also say that payment is without prejudice to your legal remedy against society for this practice
10. For ease of administration the society can take the date of grant of OC as the date of possession for all members and 18m can be computed from that date