How to add and subtract time
Time doesn’t add like ordinary decimals because it counts in sixties: 60 seconds make a minute, 60 minutes an hour and 24 hours a day. Adding 45 minutes and 30 minutes gives 75 minutes, which is 1 hour 15 minutes.
The simplest way to get it right is to turn everything into one unit, do the sum, and turn the answer back. The calculator converts both times into whole seconds, adds or subtracts them, then splits the result into days, hours, minutes and seconds. It also shows the total as decimal hours, minutes and seconds.
In Time between two times mode, enter a start and an end clock time. If the end is earlier than the start — a shift from 10 PM to 6 AM, say — the end is taken to be on the next day. Whether the time fields show AM/PM or a 24-hour clock depends on your device settings; the result is the same.
Formula
T = d × 86,400 + h × 3,600 + m × 60 + s
- d, h, m, s
- days, hours, minutes and seconds entered
- 86,400
- seconds in a day (24 × 60 × 60)
- 3,600
- seconds in an hour (60 × 60)
The result T₁ ± T₂ is split back by whole division: days = ⌊T ÷ 86,400⌋, then hours from what remains ÷ 3,600, then minutes ÷ 60; the rest is seconds. Working in whole seconds keeps the answer exact, with no rounding drift.
elapsed = end − start (+ 24 hours if end is earlier than start)
Decimal hours are total minutes divided by 60 — the figure most payroll and invoicing systems ask for.
Worked examples
Adding. 5 hours 45 minutes + 3 hours 30 minutes:
- 5 × 3,600 + 45 × 60 = 20,700 seconds
- 3 × 3,600 + 30 × 60 = 12,600 seconds
- 20,700 + 12,600 = 33,300 seconds
- 33,300 = 9 × 3,600 + 15 × 60, so the answer is 9 hours 15 minutes — 9.25 hours or 555 minutes.
A working day. From 9:00 AM to 5:30 PM: 17 × 60 + 30 = 1,050 minutes after midnight, minus 9 × 60 = 540, is 510 minutes = 8 hours 30 minutes, or 8.5 decimal hours.
A night shift. From 10:00 PM to 6:00 AM: the end is earlier than the start, so a day (1,440 minutes) is added: 360 + 1,440 − 1,320 = 480 minutes = 8 hours.
Good to know
- 8 hours 30 minutes is 8.5 hours, not 8.30. Timesheets that want decimal hours need minutes divided by 60: 15 minutes = 0.25, 45 minutes = 0.75.
- Equal start and end times give 0, not 24 hours. For a full 24-hour shift, use the add mode with 1 day or 24 hours.
- Clock changes. On the night daylight saving starts or ends, the real time between two clock times is an hour shorter or longer than the calculator shows.
- Breaks. To subtract an unpaid break from a shift, find the shift length first, then use the add-and-subtract mode to take the break away.
For whole days between dates, use the days between dates calculator; to move a date forward or back, the date calculator; and for someone’s exact age, the age calculator.
Frequently asked questions
How do I convert hours and minutes to decimal hours?
Divide the minutes by 60 and add them to the hours. 8 hours 30 minutes is 8 + 30 ÷ 60 = 8.5 hours, and 7 hours 45 minutes is 7.75 hours. The calculator shows this as ‘In decimal hours’.
How do I work out hours for a shift that goes past midnight?
Choose ‘Time between two times’ and enter the start and end as they appear on the clock. When the end is earlier than the start, the calculator counts it on the next day, so 10:00 PM to 6:00 AM gives 8 hours.
Can I enter 1.5 hours instead of 1 hour 30 minutes?
Yes. Days, hours and minutes accept decimals, so 1.5 hours is read as 5,400 seconds. The total is rounded to the nearest whole second, and seconds must be whole numbers.
What happens if I subtract a longer time from a shorter one?
The result is negative and shown with a minus sign, for example 1 hour − 1 hour 30 minutes = −30 minutes. The totals in hours, minutes and seconds are negative too.
Does the calculator allow for daylight saving time?
No. It works with clock readings only and doesn’t know your time zone or dates. On the night the clocks change, a shift really lasts an hour more or less than the result shows.
Sources
Last reviewed September 15, 2026