Time Calculator

What this calculator does

Add or take away hours, minutes and seconds, and see the new time or the new total. Clock math trips people up because time is counted in sixties. 50 minutes plus 20 minutes is 1 hour 10 minutes, not 70 of anything useful. And an evening start plus a long shift lands on the next morning. This calculator keeps track of all that for you. Give it a starting point and an amount of time, and choose add or subtract.

You can start from two things. Start from a time of day to find, say, the time 8 hours 30 minutes after 9:15, or 45 minutes before 12:10 AM. It also tells you when the answer falls on a different day. Or start from a duration (a length of time) to work out, say, 2 h 45 min plus 8 h 30 min. That's what you need to add up lap times, recordings, flights or work shifts. A duration can go below zero if you subtract a bigger amount, and the minus sign is kept.

Where it's used

Why this math matters outside the classroom:

• Adding durations. Adding 2 hours 45 minutes to 1 hour 30 minutes gives 4 hours 15 minutes.
• Crossing midnight. Adding 5 hours to 9:00 p.m. lands at 2:00 a.m. the next day.
• Video editing. Editors add clip lengths in minutes and seconds to get the running time of a project.
• Workout logs. Runners add the times of several training runs to get the week's total.

Grade level and standards: Grades 4–5; Common Core State Standards for Mathematics 4.MD.A.2, 4.MD.A.1. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

1. Choose whether you're starting from a time of day or from a duration.
2. Enter the start time (and seconds, if you need them), or the hours, minutes and seconds of the starting duration.
3. Choose Add or Subtract. Then enter the hours, minutes and seconds to add or take away. Any box can be 0, and minutes or seconds above 59 are fine.
4. Read the result. For a time of day, it tells you if the answer moves to the next or previous day. For a duration, you also get decimal hours and total minutes.

What the inputs mean

Start time

A time on the 24-hour clock. Most browsers let you type it with AM or PM, and it's converted for you.

Start seconds

Optional seconds past the minute, 0 to 59. Leave it empty for exactly on the minute.

Hours to add or subtract

Whole hours to add or take away. Big numbers work for jumps of several days: 50 hours is two days and two hours.

Minutes to add or subtract

Whole minutes. Entering 90 is the same as 1 hour 30 minutes.

Seconds to add or subtract

Whole seconds. Entering 125 is the same as 2 minutes 5 seconds.

Formula and method

seconds = hours × 3,600 + minutes × 60 + seconds
result = start ± amount   (both in seconds)
clock time: days moved = floor(result ÷ 86,400); time of day = result − days moved × 86,400
decimal hours = seconds ÷ 3,600

First, the calculator turns everything into one count of seconds. An hour is 3,600 seconds and a minute is 60. Once the start and the amount are in the same unit, adding and subtracting is ordinary math. There's no carrying between hours and minutes to worry about.

For a time of day, the seconds are counted from midnight, and a full day is 86,400 seconds (24 hours). In the formula, floor means "round down to a whole number", which gives the number of whole days moved. If the answer is a day or more, whole days are taken off and counted as days moved forward. If it's below zero, whole days are added and counted as days moved back. What's left is turned back into hours, minutes and seconds. You see it on a 12-hour clock, with the 24-hour version beside it.

Worked example

A shift starts at 9:40 PM (21:40) and runs for 5 hours 35 minutes. When does it end?

1. Start: 21 × 3,600 + 40 × 60 + 0 = 78,000 seconds after midnight.
2. Amount: 5 × 3,600 + 35 × 60 + 0 = 20,100 seconds.
3. Add: 78,000 + 20,100 = 98,100 seconds.
4. That passes midnight, so take off one day: 98,100 − 86,400 = 11,700 seconds.
5. 11,700 seconds = 3 × 3,600 + 15 × 60 + 0, which is 03:15.

Result: The shift ends at 3:15 AM the next day. In duration mode, 2 h 45 min 30 s plus 8 h 30 min 0 s gives 11 h 15 min 30 s. That's 11.2583 decimal hours.

Understanding the result

A note such as "the next day" or "2 days later" means the answer falls on a later date than the start. "The previous day" or "3 days earlier" means an earlier date. The Days moved number tells you the same thing as a number.

For payroll, use the decimal hours, not the hours-and-minutes reading. 7 h 45 min is 7.75 hours, not 7.45. This is the part most people miss.

A negative duration means you took away more than you started with. "−0 h 14 min 30 s" means the amount was 14 minutes 30 seconds bigger than the starting duration.

Assumptions and limitations

• Every day has exactly 24 hours, each 3,600 seconds long.
• Amounts are whole hours, minutes and seconds.
• A time of day is counted from midnight on a 24-hour clock.
• It ignores daylight saving time. On the night the clocks change, the answer on the wall clock will be off by an hour. Use the time zone converter when an exact date and time zone matter.
• There are no calendar dates here. To add days, weeks or months to a date, use the date calculator.
• It doesn't handle fractions of a second.

Frequently asked questions

How this calculator is checked

Method: Elementary time arithmetic in seconds (1 h = 3,600 s, 1 day = 86,400 s), with floor division to count whole days crossed. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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