Add or subtract hours, minutes and seconds to a clock time.
The time wraps around on a 24-hour clock, and the calculator shows a day offset (e.g. "+1 day") so you know the result lands on a different calendar day from the start time.
Yes — use the Subtract toggle. The same Hours/Minutes/Seconds fields are then taken away from the start time instead of added, with the result still wrapped to a valid 24-hour clock time.
The main result is on a 24-hour clock (00:00–23:59:59); the "12-Hour Format" detail converts that same moment to AM/PM for convenience, since the underlying calculation is done in 24-hour seconds.