Answer

When utilizing an FHA loan prepaid items are collected at closing to cover accrued and unaccrued hazard insurance and mortgage insurance premiums, taxes and per diem interest, and include other similar fees and charges.
The lender must use a minimum of 15 days of per diem interest in its estimate of prepaid items.
To reduce the burden on borrowers whose loans were scheduled to close at the end of the month but did not due to unforeseen circumstances, lenders and borrowers may agree to credit the per diem interest to the borrower and have the mortgage payments begin the first of the succeeding month. However, the dollar amount of the cash credit is not to be used to reduce the minimum cash investment.
For Regular No-Cash Out Refinances (Rate and Term) and Streamline Refinances, prepaid expenses may include the per diem interest to the end of the month on the new loan, hazard insurance premium deposits, monthly mortgage insurance premiums, and any real estate tax deposits needed to establish the escrow account regardless whether the mortgagee refinancing the existing loan is also the servicing lender for that mortgage.

Handbook 4155.1: 3.B.1.b, 3.C.3.a, 3.C.2.c