Countdown Timer

A deadline you can watch and share

A countdown does something a date in a calendar does not: it makes the remaining time visible and continuous. This timer counts down to any date and time in days, hours, minutes and seconds, updating every second, and it keeps the target in the address bar so the link you copy reopens the same countdown for whoever you send it to.

It suits product launches and releases, exam and application deadlines, weddings and holidays, contract expiries, and the ordinary case of wanting to know how long is left until something without doing the arithmetic in your head.

How the remaining time is worked out

The target date and time is converted to a timestamp, the current time is read every second, and the difference in milliseconds is broken into whole units: days are the total seconds divided by 86,400, hours the remainder divided by 3,600, minutes the remainder of that divided by 60, and seconds what is left.

A span of 90,061,000 milliseconds is therefore shown as 1 day, 1 hour, 1 minute and 1 second — 86,400 + 3,600 + 60 + 1 seconds. Once the target passes, the remaining time clamps to zero and the timer reports that the moment has arrived rather than counting upward into negative numbers.

The shareable link

The target and the label live in the query string, so the page URL is the countdown. Changing either updates the address without adding an entry to your browser history, and the "copy shareable link" button copies whatever is currently in the bar.

That has one consequence worth understanding: the time is interpreted in the reader’s own time zone, not yours. A countdown you set for 09:00 will read as 09:00 wherever it is opened, which is correct for something like "New Year" and wrong for something like a webinar that starts at a fixed global instant. For a fixed instant, pick a moment and tell people which zone it is in — or use the date and time converter on this site to work out the local equivalent.

The clocks change, and one of the days is not 24 hours

A countdown that crosses a daylight-saving boundary is measuring real elapsed seconds across a day that was 23 or 25 hours long, so the days-and-hours display will not agree with a count of squares on a calendar. Both are right. They are answers to different questions, and this one answers the question about time rather than the question about dates.

The target itself has a sharper version of the same problem. On the morning the clocks go forward, 02:30 does not exist — there is no such local time anywhere in that day. Most software reads it as 03:30 and never mentions the substitution. This timer counts to the instant the clock actually reaches and says in its working that the time was moved, because a target you did not set is worth being told about. The "hour that never happens" preset is there to demonstrate exactly that.

None of it disturbs the seconds. The remaining time is recomputed from the target instant on every tick rather than decremented, so a clock change, a sleeping laptop and a throttled background tab all resolve themselves the moment the page is looked at again.

Accuracy and limits

The display refreshes once a second from your device’s clock, so it is exactly as accurate as that clock is. Most operating systems keep time synchronised to within a fraction of a second, but a device with a badly set clock will show a badly set countdown.

Browsers throttle background tabs, so a countdown left in an inactive tab may update less often than once a second. It is not drifting — the remaining time is recalculated from the target each tick rather than decremented — so it corrects itself the moment the tab is visible again. There is no alarm, notification or sound when the target is reached; the timer only reports the state of the page you are looking at.