This is from the commentary to that section and then my opinion on how it is applied (I could be wrong though):
1. Crediting of payments. Under Sec. 226.36(c)(1)(i), a mortgage servicer must credit a payment to a consumer's loan account as of the date of receipt. This does not require that a mortgage servicer post the payment to the consumer's loan account on a particular date; the servicer is only required to credit the payment as of the date of receipt. Accordingly, a servicer that receives a payment on or before its due date (or within any grace period), and does not enter the payment on its books or in its system until after the payment's due date (or expiration of any grace period), does not violate this rule as long as the entry does not result in the imposition of a late charge, additional interest, or similar penalty to the consumer, or in the reporting of negative information to a consumer reporting agency.
I interpret this to mean if you receive a payment on Saturday, you can post it Monday, but the payment needs to be credited or effective dated to Saturday so they incur no additional interest on the loan. I think the language is a bit confusing because the writers were trying to help banks out since most don't have operations open on Saturdays, but the main idea is the date the bank physically receives the payment (in person, by mail, etc.) is the date the consumer must be credited. You can post it later and effective date it back to the date the payment was made, but you can't impose a posting/crediting delay that costs the consumer money in the form of late fees, additional interest, etc.
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