Time arithmetic goes wrong because time is base 60, not base 10. Adding 45 minutes to 2:30 is not 2:75, and subtracting across midnight or across a shift boundary produces answers that look plausible and are not. Doing it by hand is a common source of payroll disputes and missed connections.
The reliable manual method is to convert everything to a single unit — minutes, usually — do the arithmetic in plain decimal, then convert back. It is slower but it does not fail.
Decimal hours for timesheets
| Minutes | Decimal hours | Minutes | Decimal hours |
|---|---|---|---|
| 5 | 0.083 | 35 | 0.583 |
| 10 | 0.167 | 40 | 0.667 |
| 15 | 0.25 | 45 | 0.75 |
| 20 | 0.333 | 50 | 0.833 |
| 25 | 0.417 | 55 | 0.917 |
| 30 | 0.5 | 60 | 1.0 |
Working across midnight
A shift from 22:00 to 06:00 is eight hours, but subtracting the numbers gives negative sixteen. The fix is to add 24 hours to the end time when it falls before the start time: 06:00 becomes 30:00, and 30 − 22 = 8. The same logic applies to any duration that crosses a day boundary, and it is the single most common error in hand-calculated night-shift timesheets.
Where this comes up
- Payroll: hours worked converted to decimal, since payroll systems bill in decimal hours rather than minutes.
- Billing: professional time tracked in six-minute increments, where each unit is 0.1 hours.
- Cooking and project scheduling: adding several durations without a rounding drift.
- Travel: arrival times across time zones, where the duration and the clock change are separate adjustments.
- Training: pace and split times, which are duration arithmetic in disguise.
Rounding conventions
Employers commonly round to the nearest quarter hour, a practice permitted under US federal rules provided the rounding is neutral — it must sometimes favour the employee and sometimes the employer, not consistently one way. Rounding that always goes down is a wage violation. Where exact minutes are recorded, use them; where rounding is applied, check the direction over a full pay period rather than a single day.
Related tools
- Date Calculator — days between dates
- Online Stopwatch
- Countdown Timer
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