(How) do pilots round the time?

I'm wondering what's the time for pilots.

Given the case the watch says 10:00:20 - is it basically 10:00? And when does 10:01 start? Is it past 10:00:30 or only as the seconds move from 10:00:59 to 10:01:00?

• Welcome to the site @Qohelet. Rounding may or may not be appropriate, depending on the context. Do you mean for recording a log, or making a turn, or some other purpose? – GdD Apr 27 '18 at 8:30
• @GdD - thank you for your quick answer. I'm pretty new to aviation, so I'm not aware if and which differences exist depending on the context. I was mostly thinking of airport-related cases - for instance when you get instructions from the ATC. – Qohelet Apr 27 '18 at 8:34
• Instructions from ATC are rarely time dependent, and when they are it's to the minute, not the second. I'm sure there's a definition somewhere but realistically it's rarely important. – GdD Apr 27 '18 at 9:30
• @Qohelet If you don't agree with my edit, please rollback. I think my edit is what you meant. – Farhan Apr 27 '18 at 13:36
• For what it's worth, personally, I always just truncated the seconds if they were shown. For position reporting and the updating of ETAs we were allowed a 3 minute window, so the seconds weren't that relevant. – Terry Apr 27 '18 at 16:25

The roundings I know of in the context of US general aviation aren't strictly defined or enforced because they don't need to be (with the exception of racing; see below). The military might need to be more precise, but I have no idea about that.

Anyway, here are the main roundings a typical GA pilot might encounter:

Logging: to the nearest 6 minutes

Pilots usually log their flight time in tenths of an hour, i.e. they round to the nearest 6 minutes. There's no regulation on it and some people prefer to log in hours and minutes (maybe it's even required in some countries), but tenths seems to be most common.

ATC instructions and clearances: to the nearest minute

See this question for information on timing under IFR (some of it applies to VFR too); instructions, clearances and reports are given to the nearest minute. It is required to have a clock with a second hand but that's to make it possible to do timed turns, not because ATC gives instructions down to the second.

Air racing: to the second or less

In air racing, there are at least two ways of using time. In a speed race - like the Red Bull Air Race - you measure the elapsed time over the course. The timing for Red Bull is to one thousandth of a second.

In precision navigation air racing, you plan your flight over a specific course and calculate the time (and sometimes even the fuel) you'll require to fly it. When you fly the course, any deviation from the numbers you submitted beforehand - under or over - is penalized. Timing in those competitions is to the second.

I never think about this as there's no use cases I can think of where accuracy to the second is required in navigation or communications. ATC clearances are either not time based or are in a time block. Trying to use exact time in ATC communications would just cause confusion because not everyone's watch or clock is synchronized. Imagine if ATC told me to line up on a runway at 10:21:30, but my watch is 20 seconds fast, I could line up on a runway just as an airplane is landing!

Most of ATC communications is situational: do this now, or do this after another event; "Line up on runway 22 left", "report at point X", "cleared for takeoff". A departure clearance would read like "After departure maintain runway heading to 3000 feet, then turn left to 270 and climb to 8000 feet." There's no time mentioned, the clearance is valid from the moment received and is good until it's completed or revoked.

A flight plan is valid for a certain period, usually before and after the planned takeoff time as HH:MM. Seconds are not recorded.

If I record flight time in a logbook I don't even look at the second hand really, I generally look at the minute shown and use that. If it's past 30 seconds and I notice I may round up to the next minute, or I might not, it doesn't really matter. If I saw an event that I thought was important, like an incident, I would write down the exact time in seconds as that level of detail may be useful.