Q: Hi. I just paid my school loan in full by the due date. However, I just received a bill for additional interest. Am I obligated to pay it?

A: You likely received a bill — even after you paid your student loan in full — because your lender was probably charging you per diem interest, also known as “interim interest.” This is done primarily with mortgages, but other lenders can charge per diem interest too.

Basically, it refers to the interest charged on a loan for the day or days between the time the loan closes (or is paid off, in your case) and the end of the month.

So even though it’s a pain to get this bill after the fact, if your lender was charging you per diem interest, it is a legitimate charge and I would just go ahead and pay it.