Time calculator

How time arithmetic works

Clock time runs in base 60 for minutes and seconds, base 24 or 12 for hours, and it wraps around at midnight. That is why ordinary subtraction on a calculator misleads: 5:15 minus 3:45 is not 1.70, it is 1 hour and 30 minutes. The reliable method is to convert to minutes, do the sum, and convert back. 5:15 is 315 minutes, 3:45 is 225, the difference is 90, and 90 minutes is 1:30.

When you cross midnight, add 24 hours to the later time before subtracting. From 10:30 PM to 6:15 AM: 6:15 AM becomes 30:15 in hours past the previous midnight, 10:30 PM is 22:30, and the difference is 7 hours and 45 minutes. A good habit for shifts is to convert everything to 24-hour time first, since it removes the AM and PM step where most mistakes happen.

Hours and minutes against decimal hours

Payroll and time-tracking systems often record decimal hours, and this causes a classic mistake. 7.30 in decimal hours is 7 hours and 18 minutes, not 7 hours and 30 minutes. Those twelve minutes, repeated on every timesheet, add up quickly. These are the conversions people use most:

The sum box above shows every total in both forms. A bare number is read as minutes, and 1:30 means an hour and a half. A line starting with a minus sign is subtracted, which handles unpaid breaks: enter 8:30 and then −0:30 to get 8:00.

What daylight saving does to a calculation

Most days have 24 hours, but a day on which the clocks change has 23 or 25 in that zone. This creates three separate problems, and each one has caught out real schedules:

• Elapsed time differs from clock time. On the night the clocks go forward, a shift from 10 PM to 6 AM lasts 7 hours, not 8. On the night they go back, it lasts 9.
• Some clock times don’t exist. In New York on the spring change date, the clocks jump from 1:59 AM straight to 3:00 AM, so 2:30 AM never happens that day.
• Some clock times happen twice. On the autumn change date in New York, 1:30 AM occurs once in daylight time and again an hour later in standard time.

The calculator uses the published time zone rules for your device’s zone. “N hours from now” means N real hours, even if the clock on the wall reads differently, and any change inside the span is pointed out. For the two date-time fields, it warns you if a chosen time is skipped or repeated. To see the rule for a particular place, look at daylight saving time.

Noon, midnight and the 12 o’clock problem

“12 AM” and “12 PM” are a well-known trap. The convention on digital clocks is that 12 AM begins the day and 12 PM is noon, yet the US National Institute of Standards and Technology says the labels are ambiguous and should not be used; “noon”, “midnight” or 24-hour time say the same thing without doubt. Midnight has a second problem: a deadline of “midnight on Friday” might mean the start or the end of that day. For that case NIST suggests 0000 for midnight at the beginning of a day and 2400 for the end. If it matters, write “11:59 PM on Friday” or give the date that follows. In this calculator, 12am means the next 00:00.

Minutes ago and hours from now

For “what time was it” questions, the table above gives you the answer directly, computed from a clock we check against our server, not only your device’s time. Times can move across a date line, so the tool also tells you whether the answer falls today, yesterday or tomorrow. That matters most for spans of more than a few hours: 10 hours before 3 AM is yesterday afternoon.

Time zones

This page works in your device’s time zone. Adding hours is the same anywhere, but a clock reading isn’t: “5 PM” is a different moment in each zone. To convert a time between places, use the time zone converter, and to see the current time in your own zone with an accuracy check, use the exact time.

Which tool to use

• To count a fixed amount of time and be alerted, use the timer. To measure time as it passes, use the stopwatch.
• To plan in whole days, weeks and months, use the date calculator.
• For a countdown you can leave on screen, use days until a date, which accepts a time too.
• In a spreadsheet, times are fractions of a day: enter =(B1-A1)*24 for decimal hours, and format a sum as [h]:mm so it can run past 24 hours. For a shift that crosses midnight, use =MOD(B1-A1,1).