This kind of count — expressing a date as a single running number instead of a month-and-day pair — is often called an “ordinal date” or “day-of-year number.” January 1st is always day 1, and the count simply increases by one for every day that passes, with no reset until the calendar rolls over to a new year.
The top end of the range depends on whether the year is a leap year. In a common year, December 31st is day 365, since the year has 365 days total. In a leap year, the extra day inserted at the end of February pushes everything after it forward by one, so December 31st becomes day 366 instead. As a hypothetical: if a leap year’s February 29th fell on what would ordinarily be day 60, everything from March 1st onward that year would carry a number one higher than the equivalent date in a common year.
Ordinal dates show up more often than people expect. Manufacturers stamp them on packaging as part of “Julian date” codes so a product’s day of production can be read from a compact number. Some scheduling and inventory systems use them internally because comparing two single numbers is simpler than comparing two month-and-day pairs. They’re also a quick way to answer “roughly what fraction of the year has passed” — day 100 is about a quarter of the way through a common year, day 273 is roughly three-quarters.
The number on this page is recalculated from your device’s local date each time you load it, so it always reflects wherever — and whenever — you happen to be.