A countdown and a D-day number are not the same measurement
"D-3" is a date count: it ignores the clock and says the target is three calendar days after today. A countdown is a duration: it measures the actual gap between now and the target instant. If the target is 9am the day after tomorrow, the date count says D-2 and the duration says roughly 1 day 20 hours. Neither is wrong, they answer different questions, so this page shows both. The big boxes are the duration; the line underneath rounds it up to a whole day to give the D-number people expect.
Because the duration is real, the target time field matters. A deadline at 5pm and a deadline at midnight on the same date are seven hours apart, and if you leave the time at 00:00 you are counting to the very start of that day, not the end of it.
How the ticking works
The target is converted once to a millisecond timestamp in your device time zone. Every quarter second the display recomputes the difference from the current clock, rather than subtracting from a stored total. That distinction is the reason the numbers stay correct after a laptop sleeps, a phone locks, or a background tab gets throttled: there is no accumulating counter to drift. When you come back the very next tick is right.
Days are treated as 86,400 seconds. Across a daylight-saving change the wall-clock gap and the elapsed gap differ by an hour, so a countdown spanning a spring or autumn transition can look an hour off against a calendar. If the target is near a transition and the hour matters, check it rather than trusting the day figure.
Counting up from a past date
Put in a date that has already gone and the display flips: numbers get a plus sign and the status line says how long ago it was. This covers the "day 43 of" case, whether that is time since a project started, since a habit changed, or since anything else worth counting from. Set the time to 00:00 on the start date and the day number rolls over at midnight, which is usually what people mean when they say what day they are on.
Leaving it on screen
For a display in a room, full-screen the browser and disable the screen saver and any sleep timer; the countdown will keep running as long as the tab is alive. The tab title also carries the remaining time, so it stays readable from another tab or from a taskbar preview without switching. Refreshing the page clears the form, since nothing is saved, so a display that needs to survive a reload wants the settings typed in again rather than bookmarked.
The clock it trusts
Every figure here comes from your own device, both the current time and the time zone. If the machine clock is a few minutes out, so is the countdown, and no amount of precision in the display fixes that. Turn on automatic time synchronisation if a live countdown is going to be visible to other people. For a target in a different time zone there is no conversion field: work out the local equivalent first and enter that, because entering the remote wall-clock time directly will be interpreted as your own zone and quietly land on the wrong instant.
Questions people ask
Does the countdown stop when the tab is in the background?
Browsers throttle background timers, so it may update once a second or less often. It recomputes from the current clock each tick rather than counting down internally, so it is correct again the moment you come back with no accumulated drift.
The seconds look a little off compared to another clock.
It reads your device clock. If that is out of sync, the countdown carries the same error. Enabling automatic time setting in the operating system fixes it.
How do I count down to an event in another time zone?
Convert the event time into your own zone first and enter that. The date and time fields are always interpreted in the device time zone, so a remote local time entered directly will be off by the offset between the two.
Can I save a countdown or share a link to it?
No. There is no storage and the settings are not in the URL, so a shared link opens with the defaults. Screenshot it, or send the target date and let the other person set it up.