24 C.F.R. § 232.840

In computing loan insurance benefits, the date of default shall be considered as:

(a) The date of the lender's acceleration of the debt because of the borrower's uncorrected failure to perform a covenant or obligation under the note or security instrument; or

(b) The date of the first failure to make a monthly payment which subsequent payments by the borrower are insufficient to cover when applied to the overdue monthly payments in the order in which they become due.