Duration is a different question than "what time is it"
Clock time tells you a position on a 24-hour dial; duration tells you how much time actually passed, and those are only the same calculation when nothing crosses midnight. A recording session that starts at 23:15 and ends at 01:40 didn't last a negative amount of time, even though 01:40 minus 23:15 looks that way on paper — it ran for 2 hours 25 minutes, and duration math has to treat the day boundary as something to count through, not something that resets the clock to zero.
23:15 → 01:40 (next day)
= (24:00 − 23:15) + 01:40 = 0:45 + 1:40 = 2h 25m
Duration Between Times handles exactly this case automatically, which is the main thing separating a duration calculation from a plain subtraction.
Where "how long did this take" comes up
This kind of calculation shows up anywhere something has a start and an end and you care about the gap: a workout or run logged by start and finish time, a meeting or call that ran long, a video or podcast episode's runtime, a movie's start and end showtime, or a batch process or export that needs to finish before a deadline. None of these are payroll — they're just "how long," which is why this tool stays focused on duration and unit conversion rather than adding overtime rules or pay rates into the mix.
When a duration runs past 24 hours
Some spans aren't a single day's worth of clock time at all — a multi-day event, a long-running process, or a project timeline measured start-to-finish. The Add/Subtract and Convert modes work in raw hours, days, and weeks rather than wrapping at 24, so a 76-hour span stays 76 hours (or converts cleanly to about 3.17 days) instead of being forced into a 24-hour clock format that doesn't fit.
Calendar days vs. business days
This calculator's duration is calendar time — every day counts equally, weekends included, which is correct for measuring how long something literally took. Shipping estimates and service-level agreements often quote business days instead, deliberately skipping weekends and holidays, which is a different (and usually longer, in calendar terms) kind of count. If you need that day-counting version rather than an hour-level duration, the day counter is built for counting days between dates.
Work-hours and payroll tools
If the duration you're measuring is actually a work shift and you need it converted into pay, the time card calculator and hours calculator add overtime rules and rate calculations on top of the same underlying time math. For everyday clock-time addition and subtraction outside of a specific duration question, the time calculator is the more general tool.