Excellent.....so as the aircraft calculates in fractions if a minute then two mins 59 seconds (or 2.9 mins on somej is what we use.
The next big related flight deck myth/ mystery is that if we have active ADS reporting we do not need to manually report a revision to our time over as the system does it automatically. Once again Jep documents appear to contradict this.
Assume you are referring to ADS-C here? If so then yes, the system does do it automatically. If your onboard ADS-C downlinks an ETO change that is >2min (call it 3 min - let's not get into that debate again!) it will trigger an ETO alert on the ATC system. ATC is then obliged to challenge your ETO at the next waypoint and potentially chastise you from not updating it. A typical example is if ATC has to use (say) a 10 minute longitudinal separation standard on the route you are flying. You are at a different level to traffic ahead or behind but (say) 12 minutes apart longitudinally. You then request a level change to the same level as the other traffic. ATC approves because the separation is 10 minutes or more. Once you make the level change you encounter different winds and your onboard system calculates a revised ETO at the next waypoint that provides less than 10 minutes separation from the other traffic. At the next scheduled ADS-C update the revised ETO is downlinked to ATC. The ATC system calculates that there is no longer at least 10 minutes between you and the other traffic. The ATC system starts creating alerts, the controller has a minor heart attack, you get a sudden level change and the paperwork starts. So yes, you need to be careful that your formal position reporting ETOs are not contradicted by what your ADS-C downlinks tell ATC.