The count runs to the next December 25 and turns over at your local midnight, not on a server clock, so it always matches the calendar on your wall. The thin ring around the date is the current year filling up. For the last stretch on the day itself, start a timer.
The number is the whole days from today to the next December 25, on your own calendar. Today is the date your browser reports, taken at local midnight, so the count turns over when your date does and not when a server's does. On December 25 the page reads "Christmas is today". From December 26 it counts to the following year's Christmas, so the number never runs negative and never sits at zero for more than a day.
Nothing is counted on the server. The page is served without a number and a short script fills it in: it takes the current date, builds December 25 of the current year, rolls forward a year if that date has already passed, and divides the gap by 86,400,000 milliseconds. The division is rounded, which absorbs the one hour a daylight-saving change adds or removes, so a clock change between now and Christmas never turns a 24 into a 23. The script runs again every minute and whenever the tab comes back into view, so a page left open overnight is right in the morning.
Because the calendar day comes from your device, two people in different zones can read different numbers for a few hours around midnight, and both are right for where they are. The thin ring around the date is the current year filling up, from January 1 to the next, measured the same way.
Any date within twenty years of today has a page at radialtimers.com/countdown-to/YYYY-MM-DD, for example /countdown-to/2027-12-25 for Christmas 2027. A date the calendar does not have, like February 30, is not a page. Every countdown, this one included, also has an embed card for your own site; the snippet is below.