Time Duration Calculator

What a duration measures

A duration is the gap between two instants. You give a start and an end, and the answer is how much time passed between them. It is not a clock reading, and it is not a time zone gap, even though all three involve hours and minutes.

The difference matters when you pick a tool. Our time calculator adds several spans together, for example 1 hour 40 minutes plus 2 hours 25 minutes. Our time difference calculator compares clocks in two cities at the same moment. This page subtracts one timestamp from another, which is the question behind "how long did that take" and "how long until then".

The subtraction can cross minutes, hours, days, months and years. A meeting that starts at 9:45 and ends at 13:20 is a duration. A flight that leaves on Tuesday evening and lands on Wednesday morning is a duration. A project that starts on March 3 and ends on May 14 is a duration measured in days, not in hours on a clock.

One number, one unit at a time: the calculator reports the answer as days, hours and minutes, then converts the same span to total hours, total minutes and total seconds. Those are the same quantity written four ways, and each one answers a different follow-up question.

Subtracting times by hand, with a borrow

Subtract the columns from right to left, the way you learned subtraction with whole numbers. Seconds first, then minutes, then hours, then days. When the top number is smaller than the bottom one, borrow from the next column to the left.

Example: 9:45 a.m. to 1:20 p.m. Write the end as 13:20 on a 24 hour clock. Minutes: 20 - 45 is negative, so borrow 60 minutes from the hours: 80 - 45 = 35 minutes, and the hours drop from 13 to 12. Hours: 12 - 9 = 3. The duration is 3 hours 35 minutes.

Example across midnight: 10:40 p.m. to 1:15 a.m. Write the start as 22:40 and the end as 25:15, treating the next day as hour 24 plus 1. Minutes: 15 - 40 is negative, borrow 60: 75 - 40 = 35, and the hours drop to 24. Hours: 24 - 22 = 2. The duration is 2 hours 35 minutes, or 155 minutes.

The borrow is always one unit of the column on the left: 60 seconds make a minute, 60 minutes make an hour, 24 hours make a day. It is the same rule at every step, which is why a spreadsheet formula that adds the raw numbers instead of the time values produces nonsense.

If you subtract with a calculator instead, enter the end date and time first and the start second, or the sign comes out negative. A negative duration is a useful signal that the two fields were swapped, not a real answer.

Worked example: three calendar days apart

Start: 2:30 p.m. on October 19. End: 9:15 a.m. on October 22. The dates are three days apart, but the end time is earlier in the day than the start time, so one of those days is only partly counted.

Convert to a single unit: 2 days is 2 x 24 = 48 hours, plus 18 hours is 66 hours, plus 45 minutes is 0.75 hours, for 66.75 hours. In minutes, 66 x 60 = 3,960, plus 45 = 4,005 minutes. In seconds, 4,005 x 60 = 240,300 seconds.

The shortcut is to subtract the timestamps as one number. Convert each to seconds since a fixed reference, subtract, then divide back. The column method and the seconds method give the same 66.75 hours, and the seconds method is what every program uses because it has no borrow rules to get wrong.

The unit ladder: seconds, minutes, hours, days

Every conversion on this page is a multiplication by 60 or by 24. Learn the ladder once and the calculator becomes a check rather than a crutch.

The decimal hours column is where most mistakes happen. Six minutes is exactly 0.1 hours, 15 minutes is 0.25, 30 minutes is 0.5, and 45 minutes is 0.75. A span of 7 hours 44 minutes is 7 + 44/60 = 7.733 hours, not 7.44 hours. Writing 7.44 understates the time by about 18 minutes.

To go the other way, multiply the decimal part by 60. 0.733 x 60 = 43.98, so 7.733 hours is 7 hours 44 minutes once the seconds are rounded. The rounding is why a time sheet that converts to two decimals and back can lose or gain a minute.

The day is not always 24 hours

Clock time and elapsed time part company twice a year in most of the United States. When daylight saving time starts, clocks jump from 2:00 a.m. to 3:00 a.m., and that calendar day contains 23 hours. When it ends, clocks fall back from 2:00 a.m. to 1:00 a.m., and that day contains 25 hours. The NIST daylight saving time page explains the rule and the history behind it.

If you subtract clock readings across a spring forward day, you get one hour more than the real elapsed time. A shift that reads 11:00 p.m. to 7:00 a.m. on that night is 7 hours of real time, not 8, because the hour between 2 and 3 never happened. Across the fall back night, the same clock readings are 9 hours.

This calculator reads your device's time zone and converts both timestamps to absolute instants, so the daylight saving shift is counted correctly. The catch is that the answer depends on the time zone the browser is set to. If you are computing a duration in another country, the result is right only if your device is set to that zone.

For spans measured in whole days, the shift rarely matters. For a night shift, a flight, or a 24 hour service window, it changes the answer by an hour, which is enough to miss a deadline or misprice a shift.

When the end time looks earlier than the start

An end time earlier than the start time is normal when the span crosses midnight. A shift from 10:40 p.m. to 1:15 a.m. reads as a negative number if you only compare clock faces, but it is a 2 hour 35 minute duration if the end date is the next day.

The fix is always the date, never the clock. Put the real end date in the end date field, even when it is tomorrow, and the subtraction works. This calculator compares full timestamps, so 22:40 on Monday to 01:15 on Tuesday comes out positive and correct.

If you leave both dates the same, the calculator will tell you the end is before the start rather than silently returning a negative duration. That warning catches the two common cases: a forgotten date change, and an AM/PM swap on a 12 hour clock.

Long spans work the same way. A rental picked up at 4:00 p.m. on Friday and returned at 9:00 a.m. on Monday is 2 days 17 hours, which most rental counters bill as 3 days because any part day counts as a full day. The duration is a fact; the billing rule is a separate policy.

Rounding a duration for payroll

Payroll rarely uses the exact minute. Employers round to a fraction of an hour, and federal rules limit how far that rounding can go. 29 CFR 785.48 permits rounding to the nearest quarter hour, and the common shorthand is the 7 minute rule: up to 7 minutes can be dropped, 8 or more must be rounded up to the next quarter hour.

Rounding has to work both ways over time. The regulation says the system cannot consistently favor the employer, so a rule that always rounds down is not compliant even if it looks like a quarter hour rule. The hours worked calculator applies these rules to a shift with an unpaid break, and our time card calculator totals a full week with overtime.

Example: 8:58 a.m. to 5:12 p.m. with a 30 minute unpaid break. The raw span is 8 hours 14 minutes. Subtract the break: 7 hours 44 minutes. Rounded to the nearest quarter hour, that is 7.75 hours, and at $18 an hour the shift pays $139.50. Without rounding it pays 7.733 x 18 = $139.20, a difference of 30 cents on one shift.

Mistakes that change the answer

Most wrong durations come from four small slips, and each one has a check.

12 a.m. and 12 p.m. Midnight is 12:00 a.m. and noon is 12:00 p.m. A span from 12:00 a.m. to 12:00 p.m. is 12 hours; from 12:00 p.m. to 12:00 a.m. is also 12 hours, but only if the end date is the next day. Writing 12:00 for both ends is the single most common input error in shift math.

24 hour time without a leading zero. 1:00 in 24 hour time is 1 a.m., not 1 p.m. If the result is off by 12 hours, the input was probably read as the wrong half of the day.

Seconds that do not show. A time field that stores only hours and minutes drops the seconds. A 45 second call and a 5 second call both read as 0 minutes, and a batch of them can be off by several minutes in total.

Time zone on one end only. If the start is in Eastern time and the end in Pacific, the clock difference understates or overstates the real duration by three hours. Convert both ends to the same zone first, or to UTC, before subtracting.

The calculator handles dates, months and years, so a span from December 28 to January 3 comes out as 6 days without any month length table. It does not know about leap seconds, which the time standards bodies add to keep atomic time aligned with the earth's rotation, because those are irrelevant at the minute level this tool reports.