Date & time
Differences between dates, ages, countdowns and time zones.
Two questions a calendar is bad at, and everything here answers one of them: how far apart two dates are, and how long is left. The first is arithmetic over a system with leap years, months of unequal length and daylight saving built into it. The second is a clock that has to keep running while you look at something else.
Everything here resolves in the browser, against your own device’s clock and time zone, so the answer is yours rather than a server’s.
A gap, a duration, or a running clock
If both ends are dates you already know, you want the date calculator, days between dates, age, birthday, or one of the specific ones — drinking age, car age, decade, school grade — each of which is the same subtraction with the rule of its own domain applied on top. If one end is now and moving, you want the timers: countdown, stopwatch, pomodoro, alarm clock, world clock and the time zone converter.
Hours and the time card are under Finance, because what you actually want out of them is pay rather than elapsed time. Date range overlap and the data retention date are under Business, because those are contract questions wearing a calendar’s clothes. Epoch conversion is under Developer, where the person asking for it already is.