I remember when we had 5 minutes not 2
Time in ATC is dictated by the quote above from ANNEX 11.
I will talk of the Shannon/Shanwick interface as that is where I am familiar.
When entering the ocean from domestic airspace a number of things are happening.
The Oceanic controllers have arranged separation based on the estimated time over that you give.
We too receive a copy of the Oceanic Clearance,
Our job is to ensure that you enter the ocean in compliance with the assigned clearance.
If this is not possible we will liaise with the A/C and the Oceanic Controller to ensure separation is in place.
Either way you will enter the ocean separated from the A/C in front and behind.
When you log on to EGGX with CPDLC your avionics are updating them with your ACTUAL time over the point.
In this case your "use" of the tolerance is moot.
Modern avionics ensure more accurate position reports hence separation can be reduced (RLAT/RLONG and other wonderful acronyms).