Time: UTC, LMT, Standard Time, Time Zones and Navigation Problems
UTC, LMT and standard time
UTC is the reference time for navigation and flight plans; it is not affected by seasonal clock changes. Local mean time (LMT) depends on longitude: the Earth turns 15° per hour, so 1° = 4 minutes. LMT = UTC + longitude/15 hours, with east positive and west negative. Standard time uses a zone offset from UTC, often whole hours, and may include daylight saving. Time zones are political, so use the published zone, not a pure longitude calculation, for operational times. Zone offsets can be half-hour or 45-minute values; use the published offset rather than rounding to the nearest hour.
Time-related navigation
ETAs are calculated in UTC from elapsed time; convert to local/standard time only at the end. Crossing the International Date Line changes the date: going eastward across it, subtract one day; going westward, add one day. When a flight crosses zones, the elapsed time is unchanged; only the local clock reading changes. For a time problem, sketch a timeline: departure UTC, flight time, arrival UTC, then apply the zone offset. Keep the date attached to UTC; a local time without a date can be ambiguous.
Worked example
Departure 10:00 UTC; flight time 7 h 30 min; arrival UTC = 17:30. At 120°E the standard zone is UTC+8, so standard time = 17:30 + 8 h = 25:30, i.e. 01:30 on the next day. LMT at 045°W when UTC is 12:00: 45 × 4 = 180 min = 3 h west, so LMT = 09:00.
Traps
The sign of longitude is the main trap: east of Greenwich is ahead of UTC, west is behind. Do not apply daylight saving to UTC. Do not use LMT for ATC flight-plan times when standard time is required. Date-line errors often appear as a one-day ETA error. Remember that 1 minute of time equals 15 minutes of arc, not 1°; 4 minutes of time equals 1°.