This runs the count backwards: how many days have passed since a date already gone. It is used for anniversaries, for how long a job has been running, for the age of a document, and for the many small administrative questions that start "it's been how long?"
As with counting forward, the convention is elapsed days — the day itself is day zero and yesterday returns one. Anyone reconciling this against a period expressed in a contract should check which end is included before comparing figures, because a difference of one is almost always a difference of definition rather than a mistake in the sum.
Long intervals are where this quietly earns its keep, because leap years accumulate. Between 1 January 2000 and today the calendar has inserted an extra day roughly every four years, and mental arithmetic based on 365 drifts by a week or more across two decades. The calculator also reports the interval in years and months, which is a different figure from the day total divided by 365.25 and generally the one people mean when they say how long ago something was.
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