What time would your watch say on a summer night in Greenwich, England, if the time converts to 8 p.m. Greenwich Mean Time?
Answer: 9 p.m.
That’s not a typo. In 2016, between March 27 and Oct. 30, British Summer Time (BST) was in effect, with clocks in the United Kingdom set an hour ahead of GMT. On the last Sunday in October, at 2 a.m., they were set back one hour, and the U.K.'s time zone was back on GMT.
Take care to adjust for the switch-over when looking over weather forecasts based on Coordinated Universal Time (UTC), also called Zulu time, for an upcoming flight, and when filing your flight plans.
Need a refresher on how to apply the change? Check your home airport and destination airport’s listings in the chart supplement, where a conversion formula will be found, such as this one for Maine’s Eastport Municipal Airport: UTC–5(–4DT).
In Eastport—located on Maine’s border with the Canadian province of New Brunswick—starting on Nov. 6, a 4 p.m. (Eastern Standard Time) flight lesson translates to a flight at 2100Z. Note that just a few miles away, in Saint Stephen, New Brunswick, it will be 5 p.m. at 2100Z, because Saint Stephen is in the Atlantic Time Zone (UTC-4).
The capability of aircraft to traverse multiple time zones makes using a standard time system necessary, as the Pilot’s Handbook of Aeronautical Knowledge discusses with examples on page 16-5.
Calculating your conversions with care and making sure to correlate airport operating hours published as local times with Zulu time will keep you in sync and avoid difficulties.
Errors can cause problems, as belatedly realized by a flight instructor who explained in an Aviation Safety Reporting System report that he took off from a Florida airport without a clearance after misreading the airport’s published operating hours, leaving him under a mistaken impression that the control tower had not yet opened for the day!