Time Calculator

How to use this calculator

Choose what you want to do from the first menu. The boxes change to match:

1. Add or subtract two durations. Enter the first length of time in days, hours, minutes and seconds, choose Add or Subtract, and enter the second. Use it to total lap times or video clips, or to see how much time is left.
2. Add or subtract time from a clock time. Enter a start time and a duration to find out what time it will be (or was). The result says whether it lands on the same day, the next day or several days later.
3. Add or subtract time from a date and time. The same, with a start date, so the answer is a full date and time: “What’s the date and time 100 hours from now?”
4. Find the time between two times. Enter a start and end time. If the end is earlier on the clock than the start, the calculator assumes the span runs past midnight. Choose With dates to measure across several days.

Any box can be left at 0, and you can type decimals such as 1.5 hours. Every answer comes as h:mm:ss, decimal hours, total minutes and total seconds, followed by the working.

The formulas

The cleanest way to do arithmetic with time is to turn everything into one unit, do the math, and turn it back. The calculator uses seconds:

To convert back, divide by 3,600 for whole hours, divide what’s left by 60 for minutes, and the remainder is seconds. For decimal hours, divide the total seconds by 3,600 in one go:

Clock times work the same way, counted as seconds after midnight. When a result passes 86,400 seconds (24 hours) it rolls into the next day, and when it drops below zero it rolls back into the day before.

Worked examples

Adding two durations. 3 hours 45 minutes 20 seconds plus 1 hour 30 minutes 50 seconds. In seconds that’s 13,520 + 5,450 = 18,970. Divide by 3,600 to get 5 whole hours with 970 seconds left over, and 970 ÷ 60 is 16 minutes with 10 seconds left. The answer is 5:16:10, or 5.2694 decimal hours (316.1667 minutes).

Subtracting. 1 day 20 hours 50 minutes minus 7 hours 25 minutes leaves 37:25:00, which is 1 day, 13 hours and 25 minutes, or 37.4167 decimal hours.

Crossing midnight. A flight leaves at 9:30 PM and takes 8 hours 15 minutes. 21:30 is 77,400 seconds after midnight; add 29,700 seconds and you get 107,100, which is one full day plus 20,700 seconds. It lands at 5:45 AM the next day (05:45 on a 24-hour clock), in the departure city’s time zone.

Hours from now, with a date. Starting at 9:30 PM on Friday, November 6, 2026, add 100 hours (4 days 4 hours). The result is 1:30 AM on Wednesday, November 11, 2026. That’s five calendar days later, not four, because the extra 4 hours carry past midnight.

Time between two times. A night shift from 10:00 PM to 6:30 AM is 8:30:00, or 8.5 hours. A job that starts at 8:45 AM on November 6 and ends at 5:15 PM on November 7 took 32:30:00: 1 day, 8 hours and 30 minutes.

Minutes to decimal hours

Timesheets, invoices and payroll systems often want hours as a decimal. These conversions come straight from the calculator (minutes ÷ 60):

So 7 hours 20 minutes is 7.3333 hours, and 0.6 hours is 36 minutes (0.6 × 60). If you’re totaling a whole week of shifts with breaks and overtime, the hours calculator does that day by day.

Adding times by hand

You don’t need a calculator for short sums if you remember that you carry at 60, not 10:

• Add each column separately. 2:50 + 1:35 gives 3 hours and 85 minutes.
• Carry the extra minutes. 85 minutes is 1 hour 25 minutes, so the total is 4:25.
• Borrow when subtracting. For 5:10 − 2:45, you can’t take 45 from 10, so borrow an hour: 4 hours 70 minutes minus 2:45 is 2:25.
• Count up for elapsed time. From 9:40 AM to 2:15 PM: 20 minutes to 10:00, 4 hours to 2:00, then 15 minutes, for 4:35 in total.

Things to watch

• Midnight and noon. 12:00 AM is midnight at the start of a day and 12:00 PM is noon. The 24-hour clock (00:00 and 12:00) avoids the confusion, which is why the calculator shows both.
• Daylight saving time. In most of the US, clocks move forward an hour at 2 a.m. on the second Sunday in March and back an hour at 2 a.m. on the first Sunday in November. A period that spans one of those nights is an hour shorter or longer on the wall clock than the calculator’s 24-hour days. Hawaii and most of Arizona don’t change their clocks.
• Time zones. The calculator works in a single time zone. For a flight, add the duration in the departure zone, then convert the result to the arrival zone.
• Rounding. Durations are rounded to the nearest second, and decimal hours are shown to 4 decimal places.

For spans measured in days, weeks or months rather than hours, the date calculator counts calendar days and business days, and the age calculator finds an exact age in years, months and days. To turn hours into pay with time-and-a-half, try the overtime calculator or the paycheck calculator.

Sources

• Daylight Saving Time rules, National Institute of Standards and Technology (2026 dates, Energy Policy Act of 2005)

Updated November 2026. Results are estimates for educational purposes and depend on the inputs you provide.