Time Calculator

1. Pick a mode: Add, Subtract, or Hours between.
2. Enter the first time and the second time in hours, minutes and seconds. Blank fields count as zero.
3. For Hours between, use the 24-hour clock and add any unpaid break in minutes. The answer updates as you type.

A time calculator is a tool that does arithmetic on hours, minutes and seconds. Normal calculators count in tens. Time counts in sixties, so 45 minutes plus 30 minutes is 1 hour 15 minutes, not 75 of anything useful.

This page handles three jobs. It adds two durations, such as two video clips. It subtracts one duration from another, such as time used from time allowed. It also finds the hours between a start time and an end time, with a break taken off.

NIST lists the units it relies on: 1 minute is 60 seconds, 1 hour is 3,600 seconds and 1 day is 86,400 seconds. Those three facts are the whole method.

The tool is not a date calculator and not a time zone converter. It works inside plain clock time and does not know about calendars or daylight saving.

The calculator converts every time to seconds, does one sum, and converts back.

1. It multiplies hours by 3,600 and minutes by 60, then adds the seconds.
2. It adds or subtracts the two totals. In Hours between mode it subtracts the start from the end.
3. An end time earlier than the start means the span crosses midnight, so it adds 86,400 seconds.
4. It divides by 3,600 for hours and by 60 for minutes, and keeps the remainders.

Working in seconds avoids carrying and borrowing by hand, which is where most mistakes happen.

Three worked examples show each mode.

The last row is a night shift. The end time is smaller than the start, so one full day is added before the answer is read.

Adding to a clock time works the same way. 21:40 plus 3 h 35 min is 25:15, which the tool reads as 1:15 am the next day.

The Mode You Pick

The same two entries give three answers. 8:30 and 17:15 add to 25 h 45 min, subtract to a negative 8 h 45 min, and sit 8 h 45 min apart.

Crossing Midnight

An end time earlier than the start is treated as the next day. From 22:00 to 6:00 is 8 hours, not a negative 16.

The Break

A break comes off the span in Hours between mode. A 30 minute break turns 8 h 45 min into 8 h 15 min.

Values Over 59

Adding 90 minutes and 45 minutes is allowed. The tool carries the total into 2 h 15 min.

Decimal Entries

Entering 7.5 in the hours field means 7 h 30 min. Entering 7 hours and 50 minutes is a different time.

Clock notation and decimal hours look alike and mean different things. 7:48 is 7.8 hours, not 7.48 hours.

Pay and billing sums need the decimal form, because money is multiplied by hours. Turn hours into pay with the salary to hourly calculator. For single unit changes, the minutes to hours converter is quicker.

Totalling a Timesheet

Find each shift with Hours between, then add the shifts. Five shifts of 8 h 15 min total 41 h 15 min, or 41.25 hours.

Adding Clip, Lap or Task Times

Three laps of 1:12:45, 0:58:30 and 1:05:15 total 3 h 16 min 30 s. Add the first two, then add the third to the answer.

Finding an End Time

Add a duration to a start time and read the clock-time result. For a start time of right now, use the hours from now calculator.

Spans Longer than a Day

Hours between covers one day at most. For two calendar dates, use the date calculator and add the hours here.

1. Reading 7:48 as 7.48 Hours

The part after the colon is minutes out of 60. 48 minutes is 0.8 of an hour, so 7:48 is 7.8 hours.

2. Carrying at 100 Instead of 60

45 minutes plus 30 minutes is 75 minutes, which is 1 h 15 min. It is not 0.75 hours.

3. Mixing 12-Hour and 24-Hour Times

Entering 5 for 5 pm gives a span that runs backwards. Enter 17 for 5 pm in Hours between mode.

4. Using 12 Am and 12 Pm

NIST calls both labels ambiguous and advises against them. Write noon or midnight, or use 12:00 and 00:00.

5. Forgetting the Break

A span from 8:30 to 17:15 is 8 h 45 min on the clock. Paid time may be less once an unpaid break is removed.

6. Rounding Each Shift Before Adding

Rounding every entry first can drift from rounding the total. Add exact times, then round once if your rules allow it.

The arithmetic is exact to the second for the numbers you enter. It assumes every hour has 3,600 seconds and every day has 24 hours.

US federal rules accept recording time to the nearest 5 minutes, tenth or quarter of an hour, provided employees are still paid for all time worked over a period. That is 29 CFR 785.48(b). Your employer or state can set stricter rules.

Add the minutes first and carry every 60 into the hours. 2 h 45 min plus 1 h 30 min is 3 h 75 min, which becomes 4 h 15 min.

Subtract the start time from the end time on the 24-hour clock. From 8:30 to 17:15 is 8 h 45 min. Add 24 hours first when the end is past midnight.

Divide the minutes by 60. 15 minutes is 0.25 hours, 30 minutes is 0.5 hours and 45 minutes is 0.75 hours.

7.8 hours. 48 divided by 60 is 0.8, so 7 h 48 min is 7.8 hours, not 7.48.

Borrow 1 hour and add 60 to the minutes. 5:10 minus 2:45 becomes 4:70 minus 2:45, which is 2 h 25 min.

8 hours. From 9:00 to 17:00 is 8 hours on the clock. With a 30 minute unpaid break, it is 7 h 30 min, or 7.5 hours.

Enter the start and the end as usual. The tool adds 24 hours when the end is earlier. From 22:00 to 6:00 is 8 hours.

Neither label is safe. NIST says 12 am and 12 pm are ambiguous and should not be used. Write noon or midnight, or use 12:00 and 00:00.

Yes. Add the first two, then put the answer in the first time and add the next one. The Save button keeps each step in your browser.

No. It treats every day as 24 hours in one time zone. A shift that spans a clock change is one hour longer or shorter than shown.

No. The maths runs in your browser and nothing is sent to us. Save keeps a result only in this browser.

• NIST Guide to the SI, Chapter 5, Table 6: minute, hour and day in seconds (National Institute of Standards and Technology).
• NIST Times of Day FAQs: noon, midnight and the 24-hour clock (National Institute of Standards and Technology).
• 29 CFR 785.48: use of time clocks and rounding practices (US Code of Federal Regulations, read 2026-10-02).
• BIPM: the second, the SI base unit of time (International Bureau of Weights and Measures).