A date is not a duration
A timer answers “in ten minutes.” A countdown timer answers “how long until that moment.” You set a title and an instant. The page shows days, hours, minutes, and seconds. The instant is read in the time zone of the device you are using. Two people in two time zones who open a shared link are looking at the same clock numbers you typed, not at a converted zone. If you meant 9:00 a.m. Eastern, say so in the title. The link does not add a zone label by itself.
Copy link keeps the title and the instant in the address, using the parameters on this English page. Send it for a launch, a move-out day, a birthday dinner, or the hour tickets go on sale. Each person sees the time remaining when they open it. Nothing is stored on a server. If you open the page with no link, it suggests tomorrow on the hour, which is only a starting point you should change.
New Year and Halloween are already framed
Two dates are common enough to have their own addresses. The New Year countdown runs to midnight at the start of January 1. The Halloween countdown runs to midnight at the start of October 31, which is when the day begins, not when a party ends. After that moment passes, the page aims at the next year. Other holidays are not given English pages here. If you need a different day, type it on this countdown. If you only need the number of days and not a live clock, days between dates is the shorter tool.
At zero, the page says it is time. It does not send a system notification, because it does not ask for that permission. If you must wake up at the instant, use the alarm on your phone. For a short length later today, go back to a timer. A 15 minute warning on the day is a 15 minute timer, not a countdown you stare at for a week.
Pick the instant on purpose
Midnight is right for a new year and wrong for a delivery “sometime that day.” 9:00 a.m. is right for an opening and wrong for fireworks. Put the event in the title, not only the words “countdown,” so the link still makes sense in two months. The seconds are a spectacle. The decision, booking the trip or starting the work, happens while the day count is still uncomfortable, not at zero.
This is not a certified astronomical clock and not a legal holiday calendar. January 1 and October 31 are civil dates. Your party may start the evening before. Set the countdown to the moment you care about, and write that moment in the title. Reload without the link and you get the default again. Bookmark the copied link if you want the same target tomorrow.