Why some dates move every year
Christmas and Halloween are fixed to the calendar, but several events on this page follow rules. Western Easter falls on the first Sunday after the first full moon on or after March 21, which places it anywhere from March 22 to April 25. US Thanksgiving is the fourth Thursday of November, so it can land on any date from the 22nd to the 28th, and Black Friday follows it.
The Islamic calendar is lunar and about 11 days shorter than the solar year, which is why Ramadan and both Eids move earlier each year and pass through every season over roughly 33 years. Dates here follow the Umm al-Qura calendar. The day observed where you live depends on moon sighting and may differ by a day.
Whose midnight the timer counts to
Each countdown targets the local time on your device, not one global moment. New Year therefore arrives in Kiribati about 26 hours before it reaches American Samoa, and two friends in different countries sharing the same link will see different numbers. For a single live moment such as a product launch or webinar, tell people the time zone as well, because this tool treats the time you set as local to each viewer.
Making one of your own
Give it a name and a date between 1900 and 2100, and optionally a time. Leave the time blank to count to the start of that day. Everything is stored in the link itself, so there is no account, and whoever opens it gets the same countdown. If the date has already passed, the timer counts up to show how long ago it was, which works for anniversaries or a run of days without smoking.
When a timer looks wrong
The display relies on your device clock, so a phone set several minutes off will be off by the same amount. Browsers also slow down timers in background tabs to save power. The numbers are calculated from the actual time on each update, so the countdown corrects itself as soon as you return to the tab.