How this calculator works
Both readings are converted to minutes after midnight, so 09:00 becomes 540 and 17:30 becomes 1,050. Subtracting gives 510 minutes. When the end is earlier than the start, the reading is treated as the next day and 1,440 minutes are added, which is why 23:45 to 01:15 is 90 minutes rather than a negative number. The 12-hour clock is converted by treating 12 am as midnight, 12 pm as noon, and adding 720 minutes for the other pm hours. The table shows the answer under both the same-day and next-day assumptions so you can see what changes.
Formula
minutes after midnight = 60 x hour + minute, with 12-hour values adjusted by noon or midnight. duration = end - start, plus 1,440 when the end is earlier. decimal hours = minutes / 60.
Worked example
On the 24-hour clock, 09:00 to 17:30 is 510 minutes, or 8.5 hours and 08:30:00. 22:15 to 01:45 is 210 minutes, or 3.5 hours, because it is treated as the next day. On the 12-hour clock, 11:45 pm to 1:15 am is 90 minutes, or 1.5 hours.
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Frequently asked questions
Why does this not ask for a date?
A clock duration deliberately ignores dates and time zones: the question is how long from one reading to another, usually within a shift. When the readings span dates and daylight-saving changes, use the time duration calculator, which takes full dates and UTC offsets.
How is 12:00 handled?
12 am is midnight and 12 pm is noon. The conversion takes the hour modulo twelve, so the twelve o'clock readings land at zero and 720 minutes rather than at 1,440.
What if the two times are equal?
The same-day answer is zero and the next-day answer is 24 hours. Choose whichever matches what you mean; a shift that runs exactly one day is unusual, so zero is the default reading.
Does a daylight-saving change affect the answer?
Not in this page, because no date or time zone is involved. On a spring-forward night an 8-hour wall-clock span can be 7 real hours, and on a fall-back night it can be 9. The time duration calculator handles those cases explicitly.