Time Calculator

Three calculators on this page

They are separate tools, each with its own button and its own answer, so you can use one without clearing another.

1. Time Calculator — add or subtract two durations. Enter each in days, hours, minutes and seconds (a blank box counts as zero), choose add or subtract, and press Calculate Time.
2. Add or Subtract Time from a Date — start from a date and a clock time and move forward or back by a duration. It gives the resulting date and time, and says how many days the shift crossed.
3. Time Expression Calculator — write the whole sum in one box, such as 1d 2h 3m 4s + 4h 5s - 2030s.

Writing a time expression

Every value needs a unit straight after it: d for days, h for hours, m for minutes and s for seconds. Values sitting together are added, and the only operators are + and −, so there is no precedence to get wrong. 1h 30m is ninety minutes; 1h - 30m is thirty.

A bare number is the one thing it will not accept — 90 could be ninety of anything, so it asks for 90m instead of guessing.

Adding time to a date

Adding hours to a clock time is the part people get wrong, because the day rolls over at midnight rather than at 100. Two hours after 11:00 PM is 1:00 AM the next day — so the second calculator reports the resulting date as well as the time, and states how many days it crossed rather than leaving you to notice.

Going backwards works the same way: two hours before 1:00 AM is 11:00 PM the previous day. The time of day is always a real time between 00:00:00 and 23:59:59 — never a negative one.

How duration arithmetic works

Time is awkward to add by hand because it runs in base 60, not base 10 — 60 minutes roll over into an hour, not 100. The calculator sidesteps that by converting each duration to a single number of seconds, adding or subtracting those, and then breaking the total back into days, hours, minutes and seconds so the carrying is always right.

A worked example

Add 2h 30m and 1h 45m. As seconds that is 9,000 + 6,300 = 15,300 seconds; broken back down, the answer is 4h 15m — not 3h 75m. The minutes past 60 correctly carry into an extra hour, which is the step that trips up mental arithmetic.

Subtracting durations

Subtraction works the same way. 5h 15m minus 2h 50m is 18,900 − 10,200 = 8,700 seconds, or 2h 25m. If the second duration is larger than the first, the result is negative and shown with a minus sign rather than wrapping around.

Time unit conversions

How this differs from the hours calculator

This tool adds and subtracts durations — spans of time with no particular start or end. To work out time elapsed between two clock times, such as a shift from 9:00 to 17:30 with a break, use the hours calculator instead. To count days between calendar dates, use the date calculator.