How it's calculated
days = ⌊(Dec 25 − today) ÷ 1 day⌋
today = chosen start date, Dec 25 = next Christmas (Jan 7 for Orthodox, or a custom date). The gap is also split into weeks, months, hours and minutes.
Common questions
Does it count to Orthodox Christmas?
Yes. You can switch the target to January 7, which is when many Orthodox churches celebrate Christmas.
What happens after December 25?
The countdown rolls over to next year's Christmas automatically so it always shows the next one.