The post I wrote generally applies to PPD SEC codes, which are generally used when debiting a consumer bank account.
The 60-day rule applies to PPD SEC codes.
There's another ACH SEC code called CCD, which can be used when debiting a corporate bank account. For CCD codes, an 'unauthorized' return must be received within 48 hours, instead of the 60-days.
Here's a full list of ACH SEC codes:
"... no later than the opening of business on the banking
day following the sixtieth calendar day following the
settlement date of the original entry"
So, settlement date is the next day (mostly). + 60 days, then the next morning that banks are open. So, if 60 days is Thursday, Christmas Eve, then the next Monday morning is the cutoff.