How to calculate H1B 60 days grace period?

My Last working day was 29th Dec 2020 With last employer.

Does H1B 60day grace period starts from LAST WORKING DAY (29th Dec 2020) ?

or based on LAST PAY PAY SLIP Period end date? i.e. 12/28/2020 - 1/3/2020 ?

My new employer has filed the H1B transfer petition, But it is delayed due to Winter storm. , can I stay couple of days over the 60 days grace period while I am waiting for my H1B Transfer package to be delivered to USCIS ?