its the 2 minutes good and 3 min bad.
So if a time for the Oceanic boundary is 12:05.
We count the clearance window as
12:02:30 to 12:07:29.
not
12:02:01 to 12:07:59.
This will then cover the tolerance as per ICAO Annex 11 chapter 2.
2.25 Time in ATC.
2.25.3
Air traffic services unit clocks and other time-recording devices shall be checked as necessary to ensure correct time to within plus or minus 30 seconds of UTC.