For any given month, investments made on or before the 5th will be considered for interest calculations for that month. Interest is calculated on the lower of the balance held on the 5th of a month to the end of the month.
For e.g. An account held Rs 1 lakh at the start of September. The account holder decided to invest Rs 50,000 on Sep 10. In this case, the balance on the Sep 05 was Rs 1 lakh and was Rs 1.5 lakh at month-end. Here, Rs 1 lakh is the amount that will be considered for calculation of interest. The additional investment of Rs 50,000 would be considered for the month of October.
If, however, the account holder had deposited the additional Rs 50,000 on Sep 03, the balance on the Sep 05 would have been Rs 1.5 lakh. This would have been the amount considered for interest calculations for the month of September.