Identify whether dates are known

If a schedule includes real dates, calculate from those timestamps in software that also knows the relevant time zone. Clock mode is for a simpler case: two readings assumed to use the same local clock and separated by no more than one midnight. It cannot distinguish a Monday start from a Wednesday end, so do not use the rollover option to represent multi-day intervals.

Convert both readings

Represent each reading as seconds after midnight using hours × 3,600 + minutes × 60 + seconds. A start of 23:30:00 is 84,600 seconds, while 01:00:00 is 3,600 seconds. Clock hours must be 00 through 23, and minutes and seconds must be 00 through 59, which prevents a duration such as 25:00:00 from being silently treated as a clock.

Add one day only when selected

First subtract start seconds from end seconds. The example produces 3,600 − 84,600 = −81,000 seconds. If next-day rollover is enabled, add 86,400 seconds once, giving 5,400 seconds or 01:30:00. If rollover is disabled, preserve −22:30:00 instead. Equal start and end readings are treated as zero elapsed time, not an assumed 24-hour interval.

Do not hide DST

A day is not always 86,400 real seconds in a location that changes its clocks. From 01:30 to 03:30 during a spring-forward night may represent only one elapsed hour; a repeated fall-back hour can make the inverse difference. Because clock mode has neither a date nor an IANA time zone, it intentionally does not guess. Use the meeting time-zone planner or a calendar system for transition dates.

Use a shift example

Suppose a shift begins at 21:55:30 and ends at 06:10:15 the next day. End minus start is negative, so add one day before normalizing. The result is 29,685 seconds, which is 08:14:45. This is raw elapsed clock time; subtract unpaid breaks separately and follow the employer's required rounding policy rather than assuming the calculator applies payroll rules.

Resolve unexpected negatives or 24-hour gaps

A negative answer usually means rollover is off or the readings were entered in reverse order. A result near 24 hours often means rollover was enabled for a pair that was intended to be same-day. Confirm that both values use the same clock convention and convert 12-hour times correctly: 12:00 AM is 00:00, while 12:00 PM is 12:00. Add real dates whenever the schedule spans more than one midnight.