Time Calculator

How to do time calculations

There are two tools in one. Pick the one that fits your question:

• Clock calculator: start from a time of day (it is filled with the current time), choose Add time or Subtract time, and enter hours, minutes and seconds. You get the resulting time in 24-hour and 12-hour form.
• Date & time calculator: start from a date and a time, then add or subtract any mix of years, months, weeks, days, hours, minutes and seconds. You get the resulting date and time.

Leave a box empty when it does not apply. At least one amount must be filled in, and all fields accept decimals, although only hours keep their fractional part (see below).

The clock calculator

Time of day wraps around at midnight, so adding enough hours crosses into another day. When that happens the result also shows the calendar date, counted from today, with a plain-English note: the next day, the day prior, a number of days from now, or a number of days ago.

Example, adding: 8:45 plus 5 hours 30 minutes is 14:15, or 2:15 PM. Example, subtracting: 2:10 minus 4 hours 30 minutes goes back past midnight to 21:40 on the previous day. And 90 hours is 3 days and 18 hours, so adding it to 10:30 gives 4:30 AM four days later.

Decimals are handled the way people usually mean them. Hours are converted at 60 minutes each, so 1.5 hours is 90 minutes and 2.3 hours is 2 hours 18 minutes. Minutes and seconds are cut to whole numbers, so 1.5 minutes counts as 1.

The date and time calculator

Enter a starting date and time (an empty time means midnight) and the amount to move by. Years, months, weeks and days are whole numbers: a weeks value is multiplied by 7 and then treated as days. The answer is a full timestamp in the form year-month-day hours:minutes:seconds.

Example: 12 June 2026 at 09:00 plus 3 weeks, 2 days and 5 hours. Three weeks and two days make 23 days, which takes the date to 5 July, and 5 hours move the clock to 14:00, so the result is 2026-07-05 14:00:00.

Months are not all the same length, so month arithmetic can overflow. Adding one month to 31 May 2026 asks for 31 June, which does not exist, so the answer rolls on to 1 July 2026. In the same way, adding one year to 29 February 2024 gives 1 March 2025 (a non-leap year), while adding four years lands on 29 February 2028.

Commonly used time units

A longer example: 1,000 hours is 41 days and 16 hours (1,000 ÷ 24 = 41 remainder 16), so 1,000 hours after midnight on 19 October 2026 is 16:00 on 29 November 2026. To switch between units, the time converter and the minutes to hours converter do the multiplication for you.

12-hour and 24-hour clocks

• Midnight is 12:00 AM on the 12-hour clock and 00:00 on the 24-hour clock. Noon is 12:00 PM, or 12:00.
• From 1:00 PM onwards, subtract 12 to go from 24-hour to 12-hour time, so 15:30 is 3:30 PM. Add 12 to PM times to go the other way.
• The first hour after midnight reads 12:xx AM, not 0:xx AM, which is the most common slip when converting.

The 24 hour to 12 hour converter and the military time converter cover these conversions on their own.

Limits and common mistakes

• No time zones or daylight saving: all results are plain clock arithmetic. To compare places, use the time zone converter.
• Decimal hours are not the same as hours and minutes. 1.5 hours is 1 hour 30 minutes, not 1:50. The decimal to time converter converts timesheet-style decimals.
• Subtracting a larger amount than the starting time does not give a negative time; it goes back into the previous day.
• For paid hours between a start and an end time, the time card calculator subtracts breaks as well.
• To count working days only, see the business days calculator.

A short history of keeping time

Our split of the day into 24 hours of 60 minutes, each of 60 seconds, mixes two ancient habits. Egyptian astronomers divided day and night into twelve parts each, and Babylonian scholars worked in base 60, which later gave minutes and seconds their size. Mechanical clocks of the fourteenth century made hours of equal length practical for everyday life.

Today the second is defined by physics rather than astronomy. Since 1967 it has been the duration of 9,192,631,770 periods of the radiation from a transition in the caesium-133 atom, and Coordinated Universal Time (UTC) is kept in step with the Earth's rotation by occasional leap seconds.