Find the duration between two times of day, or add/subtract hours, minutes, and seconds from a time.
If the end time is earlier than the start time, it's treated as being the next day.
The duration mode converts both times into total seconds since midnight, then finds the difference — rolling over to the next day automatically if the end time is earlier than the start time. The add/subtract mode does the reverse: it shifts your starting time forward or backward by the exact number of hours, minutes, and seconds you enter, wrapping around midnight as needed.
Time math trips people up more often than it should, mostly because it isn't base-10 like the rest of arithmetic — 60 seconds to a minute, 60 minutes to an hour, and a day resets back to zero at midnight. Adding "1 hour 45 minutes" to "40 minutes" isn't as simple as adding decimals; you have to carry over properly once the minutes pass 60, which is exactly the kind of small error a calculator avoids.
Figuring out how long a meeting, shift, or flight actually lasts from its start and end time; working out what time you'll arrive if you leave now and the trip takes a certain number of hours and minutes; or totaling up time spent on a task when you only have a start and stop time. All of these are really the same underlying operation — either finding a difference or shifting a point in time.
Whenever a calculation crosses midnight — like a night shift starting at 10 PM and ending at 6 AM — the "end" time is technically on the next calendar day, even though only a time of day (not a date) was entered. This calculator assumes that whenever the end time is earlier than the start time, it belongs to the following day, which covers the most common real-world case.