December 25, counted to the second — the big number is the sleeps.
No — the big number is the sleeps: on Christmas Eve it says 1, on the day itself it says so. The live line underneath is the literal time left to midnight, for settling arguments down to the second.
The countdown quietly rolls over to next Christmas — 364 days, give or take a leap day. No button to press.
Entirely in your browser. Dates you type are never sent anywhere — there is no server doing the math, no analytics watching you do it.