Time Duration Calculator

Both times are converted to minutes past midnight and subtracted. From 9:00 AM to 5:30 PM that is 1050 − 540 = 510 minutes, or 8 hours 30 minutes. The same span comes back in three readings: hours and minutes, decimal hours, and total minutes. Reach for total minutes whenever you need to add several spans together — minutes add without carrying, while 45 + 45 in clock notation quietly becomes 1:30 rather than 0:90.

The most expensive confusion in this whole family of calculations: 1.5 hours is 1h30, not 1h50. The decimal part is a fraction of 60 minutes, so it has to be multiplied by 60, not read off as minutes. Worth knowing by heart: 0.25 h is 15 minutes, 0.5 h is 30, 0.75 h is 45, and 0.1 h is 6. Backwards, 20 minutes is 0.33 h and 40 minutes is 0.67 h — neither lands on a tidy decimal, which is precisely why people round them badly. Timesheets, hourly billing and consulting invoices all run on the decimal form. Entering 7.45 for a 7h45 session undercharges by 18 minutes, since 7h45 is 7.75. The calculator prints both forms so you copy the correct one.

When the end time is earlier than the start, the calculator assumes the span ran overnight and adds 24 hours. So 10:00 PM to 6:00 AM comes back as 8 hours rather than a negative number. That assumption has a hard limit: it can only add one day. A span genuinely longer than 24 hours — a 30-hour journey, a two-day event — cannot be expressed here, because the tool knows times of day and not dates. For anything covering more than a single night, work from full dates using the days-between-dates calculator.

The 12-hour clock has one truly ambiguous hour, and it ruins deadlines. 12:00 AM is midnight, the very start of a day, and 12:00 PM is noon — not the other way round. So "due by 12 AM Friday" technically means the instant Thursday ends, which is almost never what the person writing it meant. The 24-hour clock has no such trap: 00:00 is midnight, 12:00 is noon. This calculator accepts either format in either field, so you can enter 5:30 PM in one and 17:30 in the other and still get the right span.

On the nights the clocks change, elapsed time is not the difference between the two clock readings. On the night the clocks spring forward, a shift from 10:00 PM to 6:00 AM is only 7 real hours, because one hour of clock time never occurs. On the night they fall back, the same shift is 9 hours. That matters for overnight shifts, sleep tracking and anything billed through the small hours. It matters across borders too: the United States and the European Union do not change their clocks on the same dates, so for a few weeks each spring and autumn the offset between New York and London is not its usual five hours. If your span crosses a transition, work from timestamps carrying a UTC offset rather than from local clock faces.