UTC
| UTC | |
|---|---|
| Stands for | Coordinated Universal Time |
| Also called | Zulu time ("Z"); in everyday use, the same as GMT |
| Format used by hams | 24-hour clock, e.g. 1430 UTC or 1430Z |
| Daylight saving | None; UTC is the same all year |
| US offsets (standard / daylight) | |
| Eastern | UTC−5 (EST) / UTC−4 (EDT) |
| Central, including Chicago | UTC−6 (CST) / UTC−5 (CDT) |
| Mountain | UTC−7 (MST) / UTC−6 (MDT) |
| Pacific | UTC−8 (PST) / UTC−7 (PDT) |
| Time signals | |
| Stations | WWV (Colorado), WWVH (Hawaii) |
UTC, or Coordinated Universal Time, is the single time standard hams use for logs, schedules, contests and propagation forecasts. A contact between Chicago and Japan happens at one moment but at two very different local times, often on two different dates; recording it in UTC removes the confusion. UTC is the modern successor to Greenwich Mean Time, it is the same everywhere on earth, and it never changes for daylight saving time.
What UTC is
UTC is kept by atomic clocks around the world and coordinated internationally. The abbreviation is a compromise between the English and French word orders, so it reads the same in every language. For most of its history, UTC has had an occasional leap second added to keep it within a second of the earth's rotation, but timekeeping authorities have agreed to stop adding them by 2035.
The military and aviation call UTC Zulu time, from the letter Z in the phonetic alphabet, and hams often do the same: "1430Z" is said "fourteen thirty Zulu". In everyday use GMT means the same thing, but the time in London is not UTC all year, since the UK moves its clocks forward an hour in summer.
Why hams use it
- Contacts cross time zones. Both stations log the same time and date, so logs can be matched for confirmations; see QSL cards and logging.
- Events are scheduled in UTC. Contests, DXpeditions and international nets publish their times in UTC. Field Day begins at 1800 UTC, which is 1 pm in Chicago in June.
- Forecasts and predictions use it. Propagation reports, solar data and satellite pass predictions are usually given in UTC.
- Digital modes depend on it. FT8 stations transmit in 15-second slots tied to the clock, so the computer's clock must be within about a second of the correct time.
Converting from local time
To get UTC from local time in the US, add the number of hours your zone is behind UTC. Chicago is on Central Standard Time (CST), UTC−6, in winter, and on Central Daylight Time (CDT), UTC−5, in summer. So:
- 9:00 am CST in January is 1500 UTC (add 6 hours).
- 9:00 am CDT in July is 1400 UTC (add 5 hours).
Other zones work the same way: add 5 or 4 hours for Eastern, 7 or 6 for Mountain and 8 or 7 for Pacific, the first figure in winter and the second in summer. Alaska is UTC−9 in winter and UTC−8 in summer, Hawaii is UTC−10 all year, and most of Arizona stays on MST, UTC−7, all year. US daylight saving time runs from the second Sunday in March to the first Sunday in November. Europe changes its clocks on different dates, so for two or three weeks in March, and for about a week around the start of November, the gap between US and European local time is an hour different from usual, one more reason to schedule in UTC.
Hams write UTC on the 24-hour clock as four digits with no colon, from 0000 (midnight) to 2359. 1:00 pm is 1300 and 7:30 pm is 1930.
Date rollover
The UTC date changes at 0000 UTC, which in Chicago is 6 pm CST or 7 pm CDT. An evening contact on a Tuesday is therefore logged with Wednesday's date, and logging the local date instead is a common reason for a confirmation failing to match. Contests that start at 0000 UTC on Saturday begin on Friday afternoon or evening across the US, and Parks on the Air counts activations by the UTC day, so an evening activation can fall across two of them.
Keeping accurate time
Most hams keep a station clock set permanently to UTC and let their computers set themselves from internet time servers. The time-signal stations WWV, near Fort Collins, Colorado, on 2.5, 5, 10, 15 and 20 MHz, and WWVH, on Kauai, Hawaii, announce UTC every minute. Canada's CHU did the same on 3.330, 7.850 and 14.670 MHz until its shortwave service ended in June 2026. Hearing which of them is audible also gives a quick check on propagation. A phone's world-clock app can show UTC alongside local time.