Time Duration Calculator: How It Works
Adding and subtracting times in hours and minutes trips people up because time is base 60, not base 10. Ninety minutes is 1:30, not 1.90 — and confusing the two is the most common error in timesheets and billing.
Base 60 arithmetic
To add 2:45 and 1:30: add minutes (45 + 30 = 75), then carry 60 of them as an hour, leaving 15. Add hours (2 + 1 + 1) = 4. Result 4:15.
To subtract 1:45 from 3:20, borrow: 3:20 becomes 2:80, then 80 − 45 = 35 minutes and 2 − 1 = 1 hour. Result 1:35.
Decimal hours
Payroll and billing systems usually work in decimal hours, where minutes are expressed as a fraction of an hour.
| Time | Decimal | Time | Decimal |
|---|---|---|---|
| 0:06 | 0.10 | 0:36 | 0.60 |
| 0:15 | 0.25 | 0:45 | 0.75 |
| 0:20 | 0.33 | 0:50 | 0.83 |
| 0:30 | 0.50 | 1:00 | 1.00 |
Convert by dividing minutes by 60. So 7 hours 20 minutes is 7 + (20 ÷ 60) = 7.33 hours. Entering 7.20 instead — the intuitive-looking mistake — understates the time by nearly eight minutes, and across a month of timesheets that is meaningful money.
Crossing midnight
A shift from 22:00 to 06:00 gives a negative result by naive subtraction. Add 24 hours to the end time: 30:00 − 22:00 = 8 hours. Any shift where the end time is earlier than the start needs this adjustment, and forgetting it is the standard bug in shift-tracking spreadsheets.
Rounding conventions
| Increment | Used in |
|---|---|
| Exact minutes | Modern digital time tracking |
| 6 minutes (0.1 hour) | Legal and professional services billing |
| 15 minutes | Payroll and shift work |
| 30 minutes | Some consulting and trade billing |
Rounding must be applied consistently and to the nearest increment rather than always upward — always rounding up is, in many jurisdictions, a wage violation rather than an accounting convention. Note also that rounding each entry separately accumulates differently from rounding the daily total; agree which applies before invoicing.
Breaks
Whether breaks are paid varies by jurisdiction and contract. Short rest breaks are commonly paid and included; longer meal breaks are commonly unpaid and deducted. A shift of 09:00 to 17:30 with an unpaid 30-minute lunch is 8 hours worked, not 8.5 — and this is one of the most frequent sources of timesheet disputes.
24-hour and 12-hour notation
Midnight is 00:00 at the start of a day and 24:00 at the end of the previous one — the same instant with two valid labels. In 12-hour notation, 12:00 AM is midnight and 12:00 PM is noon, which is unintuitive enough that scheduling systems frequently use 11:59 PM or 12:01 AM to avoid the ambiguity entirely.