401k contribution question

Hi all! 

 

I know that the IRS allows us to contribute a max of $19,500 in our 401k account for the tax year 2021 but I had a quick question.  Does the IRS calculate this  "$19,500 cap" based on PAYCHECK date? Or is it based on the date that the money hits your 401k account?

 

For example, I received a paycheck on 12/31/2020.   However, the contribution didn't actually "hit" my 401k account until  1/4/2021.    So does that mean that this contribution counts as a 2020 contribution? Or does it count towards the 2021 contribution? (I've attached a screenshot as well) 

 

I'm just wanting to get a clear answer as to whether or not the IRS counts a contribution based on PAYCHECK date or the date in which the money actually shows up in the 401k trustee /administrator's side.   However, if it's based on paycheck date.... wouldn't you think it's kind of odd, since the IRS doesn't view individual paychecks from each company?  I mean presumably they don't really care if the company issues 26 vs. 24 paychecks per year or any other company-specific information, right? 

 

Any and all help would be greatly appreciated! Thank you! 🙂