A countdown to midnight is a different moment for everybody
This is the mistake, and sharing is exactly when it bites. Set a countdown to New Year at midnight, send the link round, and everybody who opens it sees a different number, because midnight in Sydney happened eleven hours before midnight in London.
Neither answer is wrong. They are two genuinely different things, and every other countdown tool silently picks one for you without saying which.
The same instant for everybody
One fixed moment in time. A product launch, a deadline, a kick-off, a rocket. Everybody sees the same number wherever they are, and they watch it reach zero at different times on their own clocks, which is correct.
The same clock time for everybody
New Year. The shop opening at nine. Everybody counts down to their own local version, and the numbers legitimately differ. Send this to somebody three time zones away and they are counting to a different instant, on purpose.
So the tool asks, and the link carries the answer.
Days are not always 24 hours long
Nearly every countdown works out the days remaining by dividing the milliseconds by 86,400,000. That is right for most of the year and wrong twice.
The day the clocks go forward is 23 hours long. The day they go back is 25. So a countdown crossing either one is out by an hour, and the symptom is subtle rather than obvious: the last day arrives at 23:00 instead of at midnight, or the counter sits on 1 for an extra hour.
Here the days are counted by walking the calendar a day at a time, which gets it right by construction rather than by luck. The hours, minutes and seconds are the remainder after that, so they cannot drift either.
Sharing it, with nothing stored
Copy the link. The title, the date, the time and the setting all travel in the part of the address after the hash.
A browser never sends that part to a server, which is what makes a shareable countdown possible with no account and nothing kept anywhere. There is no database here holding your wedding date, because there is no database.
Nothing is ticking while you are away
The countdown is not running somewhere with the tab shut. It works the remaining time out from the clock every second, so it is correct the instant you open it, whether that is in five minutes or in five months.
Which also means it cannot quietly go wrong while nobody is looking, and there is nothing to keep alive or renew. Everything happens on your own machine.
Common questions
Why does my shared countdown show a different number to somebody else?
Because it is set to count to a local clock time rather than to a fixed moment. Midnight in Sydney happens eleven hours before midnight in London, so everybody counts down to their own. That is right for New Year and wrong for a deadline, and the tool asks which one you want rather than picking for you.
What is the difference between the two settings?
The same number for everybody fixes one moment in time, so a launch or a kick-off reaches zero simultaneously wherever you are. Their own clock counts to the same clock reading in each person’s own zone, so New Year arrives for each of them in turn.
Why do days sometimes come out wrong on other countdowns?
Because days are usually worked out by dividing the milliseconds by 86,400,000. The day the clocks go forward is 23 hours long and the day they go back is 25, so any countdown crossing one is an hour out and the last day arrives at 23:00 instead of at midnight. Here the days are counted by walking the calendar.
I set a date and it counted to the start of the day.
A date with no time means 00:00, which is the very beginning of that day rather than the end of it. If you meant the end, set the time to 23:59.
How do I share it?
Copy the link. The whole countdown travels in the part of the address after the hash, which a browser never sends to a server, so nothing is stored anywhere and there is no account at either end.
Does it keep running when I close the tab?
There is nothing to keep running. It works the remaining time out from the clock every second, so it is correct the moment you open it however long the tab has been shut.
Can I count down to something that has already happened?
It shows zero and says it has happened, which is a perfectly good way of marking that something has. It will not count upwards.
Is the date stored on a server?
No, and there is no server. The date lives in the link and in your browser. Closing the page without copying the link loses it.