Yes, a workout timer can run on the Lock Screen, keep perfect time with the screen off, and show the round and countdown on the Lock Screen and in the Dynamic Island while it does. Whether the one on your phone manages it comes down to a single implementation choice you cannot see from an App Store listing: does it count seconds as they pass, or does it work out where it is from the clock.
iOS suspends an app within seconds of the screen locking. Everything about interval timers on iPhone follows from that one fact.
What happens the moment your screen goes dark
When your app leaves the foreground, the system gives it a short window to tidy up and then stops giving it CPU time. Apple’s documentation puts that window at approximately five seconds, and an app that overruns it is terminated and purged from memory. After that the app is suspended. It stays in memory, and it does not execute.
Suspended means every loop inside the app stops. A repeating timer firing sixty times a second and adding to a counter stops adding. A progress value advanced frame by frame stops advancing. The wall clock keeps moving while the app is frozen.
There is no setting that turns this off, and none of it is a bug. It is how a phone in your pocket survives an afternoon. Two background modes let an app keep doing narrow, declared work while suspended elsewhere, and audio playback is one of them, which is why interval cues can still reach your headphones. Running arbitrary code to count rounds is not one of them.
The drift, worked through in rounds
Take a standard Tabata: eight rounds of 20 seconds work and 10 seconds rest, four minutes in total. You start it, get through the first round and a bit, and at 30 seconds you lock the phone and set it on the bench. Two minutes later you pick it up.
The app got roughly five more seconds of execution after the lock, then froze. Here is where the two kinds of timer are, second by second.
| Wall clock since start | Where the session actually is | Where a ticking timer shows you |
|---|---|---|
| 0:30 | Round 2, work, just started | Round 2, work, just started |
| 0:35 | Round 2, work, 5s in | Round 2, work, 5s in, then frozen |
| 1:30 | Round 4, work, just started | Round 2, work, 5s in |
| 2:30 | Round 6, work, just started | Round 2, work, 5s in |
You wake the phone at 2:30 and the correct answer is round 6. A ticking timer tells you round 2 with fifteen seconds of work left, and then carries on from there as though nothing happened. It is four rounds and 115 seconds behind the world, and the error always runs the same direction: it hands you rest you did not earn and stretches a four-minute finisher into something else.
On a 40-minute circuit with two or three pocket checks along the way, the same mechanism turns into minutes of accumulated drift, and by then you have no idea which round is real.
The fix is to stop counting
A timer that survives suspension never counts anything. It records the wall-clock instant you pressed start, expands the whole workout into a list of phases up front, and answers every question by subtraction.
Which phase am I in? Take the current time, subtract the start instant, walk the phase list until the elapsed seconds land inside one. How many seconds are left? The end of that phase minus now. Which round is this? A property of the phase you landed in. Every one of those is a pure function of elapsed real time, with no state that can fall behind, because there is no state.
Freeze that app for two minutes, for ten minutes, or for the length of a phone call, and it returns to precisely where the clock says it should be, because the answer was always a calculation on the current time. This is how Rounds works, and it is the reason its timer engine has no ticking loop anywhere in it. Audio cues are scheduled against that same timeline in advance, so the beeps arrive on time even while the app is asleep, which is the other half of the problem covered in the HIIT timer guide.
The visible half: Live Activities
Correct timekeeping is worth nothing if you have to pick the phone up and sign in to read it. That is what Live Activities are for, available since iOS 16.1.
A Live Activity is a small piece of your app’s interface that the system renders on the Lock Screen and, on iPhones that have one, in the Dynamic Island. The important trick for a timer is that SwiftUI can display counting text from a date interval, so you hand the system a start and end date once and it animates the digits itself. The countdown on your Lock Screen keeps moving while your app is suspended, because the app is not the thing drawing it.
Here is what a Live Activity can and cannot do for a workout timer.
| Capability | Available | Detail |
|---|---|---|
| A countdown that keeps ticking | Yes | The system renders timer text from a date range without waking your app |
| Current phase, round and set | Yes | Your app pushes a new state; static and dynamic data together cap at 4 KB per update |
| Pause, skip and stop buttons | Yes, iOS 17 and later | SwiftUI buttons and toggles driven by an App Intent. They do nothing in CarPlay |
| Fetching data itself | No | Each Live Activity runs in its own sandbox, with no network and no location access |
| Running indefinitely | No | Eight hours active, then the system ends it and clears the Dynamic Island at once |
| Lingering after it ends | Partly | Up to four further hours on the Lock Screen, twelve hours from start in the worst case |
Two limits get quoted at people who do not need them. The eight-hour ceiling is real and irrelevant to a gym session; it exists for flight trackers and food deliveries. The update budget is real too, and it applies to updates pushed from a server through APNs, which get throttled if an app is greedy. A workout timer updates its own Live Activity locally while the phase changes, and the countdown between phases redraws itself, so a well-built one sits nowhere near either limit.
Sound is a separate mechanism
Live Activities are visual only. Audio comes from declaring the audio background mode and holding an active playback audio session, which is what keeps the app producing sound with the device locked and the Ring/Silent switch flipped. An interval timer needs both halves: the audio session so you can hear the transitions with the phone face down, and the Live Activity so a glance tells you the round.
If a timer has one and not the other, you can feel it. Cues with no Lock Screen display means picking the phone up to find your place. A Lock Screen display with no reliable audio means watching the screen through the work interval, which is the interval you are meant to be spending at full effort.
Test the timer you already have
This takes four minutes and settles the question for any app on your phone.
- Set up an eight-round Tabata: 20 seconds work, 10 seconds rest, no preparation countdown.
- Press start and let round one run.
- Lock the phone at around the 30-second mark and put it face down.
- Wait two full minutes. Listen for whether the cues keep arriving.
- Wake the phone and read the round number.
At 2:30 into the session, a correct timer shows round 6. Anything showing round 2 or 3 has been counting instead of calculating, and it will do the same thing to every session you run with it. While the phone is locked, glance at the Lock Screen: a timer with a Live Activity shows the phase and a moving countdown without you touching anything.
Run that test before a session that matters, not during one. It is also the fastest way to sort a shortlist, which is roughly how the interval timer comparison is put together.
Rounds is in open beta through TestFlight at userounds.app, built so that the phase timeline is computed from the wall clock and mirrored to the Lock Screen and the Dynamic Island. Whatever you end up using, run the four-minute test on it first.
Common questions
Does an interval timer keep running when the iPhone is locked?
A well-built one does. iOS suspends the app within seconds of the screen locking, so a timer that advances a counter as seconds pass freezes and comes back late. A timer that recorded the instant you pressed start and works out the remaining seconds from the clock comes back exact, because it was never counting anything. From the outside the two look identical until you lock the phone.
Do workout timers work on the Lock Screen?
The good ones show the running countdown on the Lock Screen through a Live Activity, and on iPhones with a Dynamic Island the same information appears there. The system renders the counting digits itself from a start and end date, so the numbers keep moving while the app sits suspended. You get the phase, the round number and the seconds left from a glance at a phone lying on the bench.
Why does my interval timer show the wrong round after I wake my phone?
Because it was ticking rather than calculating. While the app was suspended its internal counter stopped, so the round it shows is the round it was in when the screen went dark, plus the few seconds iOS allowed before suspension. Lock the phone for two minutes during a Tabata and a ticking timer can come back four rounds behind. The error only ever gives you extra rest.
Can a Live Activity have buttons to pause the timer?
Yes, since iOS 17. A Live Activity can contain SwiftUI buttons and toggles wired to an App Intent, so a pause or skip control on the Lock Screen is possible without opening the app. Two limits apply: buttons and toggles on Live Activities do nothing in CarPlay, and the Live Activity itself runs in a sandbox with no network access, so the action has to be handled by your app.
How long does a Live Activity stay on the Lock Screen?
A Live Activity can be active for up to eight hours. After that the system ends it and removes it from the Dynamic Island immediately, while the Lock Screen keeps showing it for up to four more hours, which is twelve hours in total. No gym session comes near that ceiling, so for an interval timer the limit is academic. It matters for apps tracking flights or deliveries.
Do I need to keep the screen on for my workout timer to work?
No, and leaving it on to be safe is a sign the timer is built wrong. An app that declares the audio background mode and holds an active audio session keeps playing cues with the device locked, and a Live Activity handles the visual side. Test yours before you trust it: if it needs the screen awake to stay correct, it will also drain your battery through a long session.