“Clock in at 9, out at 18 — how long did I actually work?” “Started studying at 22:15 and stopped at 23:50. How many minutes was that?” “If my shift runs from 22:00 to 5:00, how long is it?” Time duration calculations show up everywhere — timesheets, study logs, cooking, travel — and they trip people up surprisingly often.
This guide explains the basic duration formula, how to handle times that cross midnight, how to subtract breaks, and the mistakes that trip people up — with worked examples throughout. A free browser calculator handles the arithmetic at the end.
The Basic Duration Formula
Duration takes one line to compute:
duration = end time − start time
For example, a task that started at 9:15 and ended at 12:45 took 12:45 − 9:15 = 3 hours 30 minutes. Because time uses base-60 arithmetic (60 minutes per hour), the safest approach is to convert everything to minutes first, then subtract.
| Step | What to do | Example (9:15 → 12:45) |
|---|---|---|
| 1 | Convert the start time to minutes | 9 × 60 + 15 = 555 min |
| 2 | Convert the end time to minutes | 12 × 60 + 45 = 765 min |
| 3 | Take the difference | 765 − 555 = 210 min |
| 4 | Convert back to hours | 210 ÷ 60 = 3 h 30 min |
The reverse pattern works the same way. A night shift that ended at 5:30 after starting at 9:00 the previous evening lasted 9:00 − 5:30 = 3 hours 30 minutes. Once the relationship between end and start is clear, the direction — morning or night — no longer changes the formula.
Keep the notation in 24-hour time
Clock times come in two notations: the 12-hour clock with AM and PM, and the 24-hour clock running 0:00 to 23:00. For calculations, stick to 24-hour notation — mixing up AM and PM leaves almost no numeric clue behind, which makes the mistake hard to spot.
What does "2 o'clock" mean?: 2 AM and 2 PM produce completely different durations. In meeting minutes and timesheet entries, recording times as "14:00" prevents confusion for anyone reviewing them later.
Tip for crossing midnight: When the end time is earlier than the start time, add 24 hours (= 1,440 minutes) to the end before subtracting. 22:00 → 29:00 makes the gap 7 hours. This is an essential technique for night-shift and overnight calculations.
Worked Examples: Three Common Situations
1. Summing work hours
A shift from 9:00 to 18:00 with a lunch break from 12:00 to 13:00:
- Total elapsed: 18:00 − 9:00 = 9 hours
- Minus the break: 9 hours − 1 hour = 8 hours worked
The step people forget is subtracting the break. And when summing multiple days, compute each day’s worked time first, then add them up — mistakes drop noticeably.
2. Study and work session logs
Studying from 22:15 to 23:50 is 23:50 − 22:15 = 1 hour 35 minutes, or 95 minutes. Logging sessions in minutes makes weekly and monthly totals easy: 95 minutes × 4 days a week = 380 minutes, about 6 hours 20 minutes.
3. Overnight and late-night shifts
A shift from 22:00 to 6:00 the next morning: think of the end as 30:00, and the duration is 8 hours. Adding 24 to the end time handles the midnight wrap-around mechanically.
If a break falls in the middle, compute the total first and subtract the break afterward: a 22:00–06:00 shift with 2 hours of breaks is 6 worked hours.
Labor treatment is a separate question: Overtime pay and late-night premiums apply multipliers to these durations, and break handling follows workplace rules. This guide covers the arithmetic only — for legal or payroll decisions, consult a labor specialist.
5. Solving for the start time
The reverse also works: if a task takes 3 hours and must finish by 18:00, the start is 18:00 − 3:00 = 15:00. Working backwards from a deadline makes it easy to plan a realistic start.
The same idea applies to meetings. A 1.5-hour meeting that must end by 17:00 starts at 17:00 − 1:30 = 15:30. One subtraction gives you a schedule that protects the end time.
Calculate Durations with a Tool
Hand calculations invite slips. The Tools Hub time duration calculator shows the duration between two times in hours and minutes as soon as you enter them, handles the midnight wrap-around automatically, and shows the total in minutes as well. Everything runs in your browser and nothing you enter is transmitted.
How to use it (3 steps)
Enter the two times
Open the time duration calculator and fill in the start and end times.
Read the duration
Hours, minutes and the total in minutes appear instantly. Cross-midnight spans are handled automatically.
Use the add/subtract mode
The shift tab adds or subtracts hours and minutes from a base time — useful for working out when a meeting will end.
Tool mentioned in this article
Time Duration Calculator
Duration between two times, computed instantly — cross-midnight friendly, free, in your browser.
For multi-day spans, the date calculator counts whole days between dates, and the stopwatch measures a session live as it happens.
Summary
- Duration = end time − start time; convert to minutes before subtracting
- When the end is earlier than the start, add 24 hours first (the midnight wrap-around)
- Worked time = elapsed time − breaks
- Overtime pay and premium calculations need labor rules on top of the arithmetic
- Tools Hub’s time duration calculator does it instantly — free, in your browser
Try it on yesterday’s work or study session: enter the start and end times and see whether your own math matches.
FAQ
How do I calculate when the end time is earlier than the start time?
Treat it as crossing midnight: add 24 hours to the end time before subtracting. 22:00 to 05:00 is 29:00 − 22:00 = 7 hours. The calculator handles this wrap-around automatically.
How do I subtract a break?
Compute the full elapsed time first, then subtract the break. A 9-hour shift with a 1-hour lunch break is 8 worked hours. If the break comes in multiple chunks, sum them before subtracting.
Does this work with seconds?
This tool measures in hours and minutes. For second-level precision, use the stopwatch tool on this site.
Are the times I enter sent anywhere?
No. All computation happens locally in your browser and nothing you enter is transmitted.
