The very first text I read stated it was based on Liability Deposit Due Date, not Paycheck date. Or, as I teach it in my Payroll Class:

Liability Period starts with the paycheck date, because that's when money happened.

Liability Due Date is based on your payment cycle per the IRS. Which can be Monthly, Quarterly, or Accelerated as next date or 3-banking dates.

If you find more specific or even more current info, please post it in the topic where I put a bunch of links already. It's nice when it is all in one place.