Your operating system feeds the browser its time. Pick UTC for a display that never shifts. Pick GMT and you are choosing a civil time zone that goes dark for half the year. From late March to late October the United Kingdom and Ireland run on British Summer Time. A GMT clock runs one hour wrong that whole stretch. UTC stays put.
What does UTC mean on a clock display?
UTC is Coordinated Universal Time, the reference every other zone offsets from. The range runs from UTC-12:00 to UTC+14:00. No daylight saving. No spring forward, no fall back. Air traffic control, the Network Time Protocol, the International Space Station and your phone's cellular time sync all reference it.
A UTC readout answers one question: what is the uncorrected time right now, everywhere, before any country applies its offset.
What does GMT mean on a clock display?
GMT is Greenwich Mean Time, the local solar time at the Prime Meridian in Greenwich, London. It is a zone, not a standard. The United Kingdom, Ireland, Portugal and several West African nations use it as civil time in winter.
The offset matches UTC. The meaning does not. A zone describes a place. A standard describes a reference. GMT is the place. UTC is the reference.
When does GMT stop matching UTC?
Every year, when the United Kingdom and Ireland switch to British Summer Time. In late March the clocks move forward one hour. GMT disappears from active use until late October. A display locked to GMT all year shows a time no country observes for roughly six months.
The dates shift slightly each year. Check the UK government's annual summer time order for the exact Sunday.
How does the system get the time for a UTC or GMT display?
The software calls the operating system. It does not fetch time from a server. Date.now() returns milliseconds since the Unix epoch: 1 January 1970 00:00:00 UTC. Not GMT. UTC.
If the OS clock is wrong, the on-screen clock is wrong. Most consumer operating systems sync via NTP. Between syncs a quartz crystal oscillator keeps time, drifting 1 to 15 seconds per month on typical hardware. No API exposes NTP status, stratum level or last sync time. You cannot check sync health from JavaScript.
When you set a display to UTC, the software applies the UTC zone to the OS time. When you set it to local, it applies the system time zone. The underlying instant is identical.
Why your stopwatch must ignore UTC and GMT
A live clock uses wall-clock time. A stopwatch must use monotonic time. performance.now() increases steadily and ignores NTP corrections, manual clock changes and DST transitions. Date.now() can jump backward if the OS steps the clock. Build stopwatches on performance.now(). Build clocks on Date.now(). Mixing them gives you elapsed times that stretch or shrink.
How to keep a UTC clock visible without the screen sleeping
Request the Screen Wake Lock API. It prevents the display from dimming or locking while the page is visible. The lock releases when the tab is hidden or the device locks. It cannot override OS power settings on all platforms. On OLED screens, combine it with subtle pixel movement to reduce burn-in risk.
Fullscreen API requires a user gesture: a click or keypress. It cannot activate on page load. On iOS Safari, fullscreen support arrived late and behaves differently from Chromium-based tools. Test on the target device.
Why does the display skip seconds in a background tab?
Apps throttle timers in hidden tabs. Chrome began aggressive throttling around 2017, dropping setInterval to 1 Hz. requestAnimationFrame stops firing entirely. When you return to the tab, the readout recalculates from system time and jumps to the correct display. The visual update paused. The time itself did not.
For a clock that stays visible, keep the tab focused or use a dedicated device in kiosk mode.
How to set up a UTC clock on a second monitor
Open a fullscreen clock on the secondary display. Set the time zone to UTC. Request wake lock so the screen stays on. If the monitor is connected to a Raspberry Pi or similar device, configure it to boot directly into kiosk mode and auto-launch the clock on startup. Test recovery after a power cut: the device should restart, reconnect and display the time without human intervention.
How to display UTC on a lobby or signage screen
Run the display from a device in kiosk mode. Set the clock to UTC if the audience spans time zones. If the audience is local, use local time. Disable screen savers and sleep timers at the OS level. Schedule a daily reboot if the device's memory management requires it. Check the display during DST transitions to confirm the OS applied the change correctly.
How to add a UTC clock overlay in OBS
Add a source pointed at a UTC clock. Size and position the overlay. Use a dark background with light text for readability at low bitrates. If you need chroma key, pick a background colour absent from the clock digits. For a global audience, add a second clock showing a major audience time zone. The overlay always shows the streamer's system time interpreted as UTC, so verify your OS clock is synced before going live.
How to time an exam or presentation with a UTC clock
Project a fullscreen countdown timer. Set the warning thresholds your exam rules require. Use a monotonic timer for the countdown, not a wall-clock countdown, so NTP corrections do not shift the remaining time. Position the display where every seat can see it. Run a second device as a backup. If the room has no internet, pre-load the page and confirm the OS clock was synced beforehand.
Skip GMT for year-round displays
Skip GMT unless you are scheduling with someone in a country on GMT during winter and you know the switchover dates cold. Skip manual clock setting: human reaction time adds 0.5 to 2 seconds of error, and drift begins immediately. Skip millisecond displays for precision work: the rendering pipeline adds 16 to 50 ms of variable latency. The digits update fast. The accuracy is casual.