Time Calculator

The time calculator tool allows two everyday tasks. You can add or subtract durations of times in days, hours, minutes and seconds, or move a date and time forward or backward by a given duration. Select a mode, enter values, and the result will be updated in real-time.

Using the Time Calculator Tool:

In the top section of the screen, select the desired operation. For Add/Subtract Time, enter two time durations and choose Add or Subtract to get the result either as a sum or total duration in one unit. For Add/Subtract Time to Date, enter a start date, start time, and the amount of time to add or subtract to get a new date, day of week, and time for that day.

Any field can be left blank; blank fields are treated as a 0 and will be included in the calculation. You can enter values into fields that aren't strictly necessary, such as adding 45 minutes or subtracting 2 hours 30 minutes.

Adding and subtracting durations of time:

Time is not added like a normal decimal number. This is because there are rollovers at 60 and 24, but not at 10. There are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a day. So adding 30 minutes to 40 minutes results in "1 hour 10 minutes", and "26 hours" is equivalent to "1 days 2 hours". The calculator tool automatically processes all these rollovers and displays the result in normal units.

If the result of a subtraction is negative - which happens when the time to subtract is longer than the starting time - then this result will be shown by the calculator and also give the actual negative difference so as not to confuse the user.

Carry-over in manual calculations of time format:

To manually subtract two times in the time format, you must first adjust hours and minutes. If the minute to be subtracted is larger than the existing minute, borrow an hour. Add 60 to the top minute and subtract one from the top hour before performing the subtraction. For example: To subtract 9:58 from 13:57, you get 12:117 minus 9:58, which results in 3 hours and 59 minutes. The calculator automatically takes care of the carryovers, but this method is useful for doing mental math quickly.

Add or subtract times to a date:

To calculate a future or past time, add or subtract days, hours, minutes and seconds from a given date and time. Common use cases are deadlines ("in 72 hours"), timers for cooking or brewing processes that span midnight, as well as arrival times after long-haul flights. The calculator will return the exact date, day of week, and new time for that day.

The time format dropdown determines how the hours of the start time are interpreted. In AM/PM mode 12 AM is midnight and 12 PM is noon. In 24 hour mode the hours are interpreted as entered. Because calculations are accurate to the second, adding a time that crosses midnight will correctly roll over to the next day, and subtracting a time that crosses midnight will correctly roll back to the previous day.

Units of time

Since time can be expressed in various units, it is convenient to summarize the conversion relationships in one place. The second is an SI base unit and all larger units are fixed multiples of the second (the length of calendar months and years varies so average values are given here).

Why time is calculated in a 60-system

The division of the minute into 60 seconds and the hour into 60 minutes comes from the sexagesimal system (base 60) of the ancient Sumerians, later adopted by the Babylonians. Sixty is an excellent highly composite number; it has twelve divisors, so that an hour can be evenly divided in half, thirds, quarters, fifths, sixths, tenths and twelfths with no remainder. This convenience led to this system surviving longer than almost any other unit reform, and is the reason why angles and coordinates are still measured in the same way today.

The 24-hour system has a different origin. The Egyptians divided the day into twelve sections using sundials, and they also divided the night by counting the rising of stars in twelve parts, for a total of twenty-four divisions. These lengths varied with the seasons until Greek astronomer Hipparchus proposed equal-length "equinoctial" hours around 130 BCE. It was not until mechanical clocks became available in the fourteenth century that fixed units of time became common.

A short history of timekeeping.

Devices to measure time developed from sundials and water clocks (clepsydras) to candle clocks and sand clocks that divided the day for work or prayer. In 1656, Christiaan Huygens built the first pendulum clock, which had an accuracy of about ten seconds per day. This was a major advance at the time. Today, the SI second is defined by the emission of cesium atoms, and cesium atomic clocks have measurement errors within one second over millions of years.

For most everyday planning purposes however simple arithmetic operations with days, hours, minutes and seconds are sufficient. That is exactly what this calculation tool processes.

Note:

Leave any unused fields blank. Blank fields will be calculated as 0. Use the "Subtract" option and do not enter negative numbers. Be aware of AM/PM or 24 hour selection when adding time to a date. This is the most common cause for an off by one hour result. Switching between seconds, minutes, hours, and days total display allows you to read the results in your preferred format.

This calculator is for general planning and educational purposes. If it concerns legal deadlines, medication schedules or changes in time zones or daylight savings time, please verify the results with a reliable clock as this tool uses fixed units independent of any time zone.