How many days until (or since) a date
Calculate how many days remain until a target date, or how many days have passed since it.
days remaining = target date − as-of date
What's the difference between this and Date difference?
Countdown is built around a fixed target date and "how many days from now" framing (with a today default for the starting point). Date difference is symmetric — pick any two dates to compare.
Can I count down to a date that has already passed?
Yes — the result switches to showing how many days ago that date was, instead of days remaining.