Classroom timer or interval timer?

The difference is repetition, not duration

A classroom timer is a single countdown with a visible end. Five minutes for the starter task, twenty for the test, ninety seconds to pack up. Its whole job is to be unambiguous from a distance: one number, large, projected, with nothing else competing for attention. Nobody watching it needs to know how much of the lesson is left, only how much of this is.

An interval timer repeats. Work, rest, work, rest, for a fixed number of rounds, and the thing you actually need on screen is which phase you are in and how long is left of it — plus, secondarily, how many rounds remain so you can pace effort across them. A single countdown cannot express that, which is why running intervals on a classroom timer means resetting it by hand between every round, and why the round count is the first thing you lose track of.

The failure people hit: a backgrounded tab

This is the one that catches people out, and it only bites the repeating timer. Browsers throttle JavaScript timers in a background tab — typically to about once a second, sometimes far less on a laptop running on battery. A timer built by counting its own ticks therefore runs slow while displaying a confident number, and you can finish a twenty-minute session having actually trained for seventeen with the round count insisting otherwise.

A single countdown mostly survives this, because you are usually looking at it. A twenty-minute interval session usually is not the foreground tab — you switch to music, or the screen sleeps. The fix is not to trust the tick: read elapsed wall-clock time on every frame instead, so a throttled tab simply catches up the instant it is visible again. That is how the interval timer here is built, and it is worth checking in whatever timer you use — set a five-minute round, switch tabs for five minutes, and see whether it agrees with a clock.

One thing no browser timer can promise: audio on time in a background tab. If you are going by sound rather than sight, keep the tab visible.

Sound, and why neither one flashes

Both timers mark transitions with a short tone rather than a visual flash, and both let you switch it off. That is a deliberate constraint rather than a missing feature. A timer is watched continuously — by a room full of children, or by one person mid-effort — which is precisely the situation that full-screen colour flashing is a hazard in. Held colour states and a brief tone carry the same information without strobing anyone.

The practical consequence is worth planning around. A classroom timer usually has the room's attention already, so the tone is a courtesy. An interval timer is often the thing telling you to stop when you are not looking at the screen, so the tone is the interface — and browsers will not play audio at all until you have interacted with the page, which is why the first press is what unlocks it. If a session needs to be silent, both work fine muted; you just have to be watching.

What to check before you rely on either

For a projected timer, the only test that matters is reading it from the furthest seat, which is a question about pixel height rather than font size — a browser at 100% zoom on a 4K projector renders type smaller than the same page on a laptop. Put it fullscreen, walk to the back, and adjust before the lesson rather than during it. Check the exit too: a fullscreen timer you cannot dismiss on a tablet is worse than no fullscreen at all, so confirm Escape or a tap brings you back.

For intervals, check that your protocol is the protocol. Tabata is a specific set of numbers — 20 seconds of work, 10 of rest, eight rounds, four minutes — and not a general word for interval training; a tool offering "Tabata" at 30/15 is offering something else under a borrowed name. EMOM is a minute-long round where the rest is whatever remains after the work, so a tool that insists on a separate rest phase is not doing EMOM either. Both are set correctly here, and both are worth verifying in anything else you use.

Finally, the honest limit that applies to both. Neither timer knows whether anything happened. They count time, not effort or attention, and they will happily run a full session in an empty room. That is a feature — an instrument that reports a measurement and makes no claim about you — but it does mean the timer is never the thing keeping the session honest.