Time Calculator
You can "add" or "subtract" two time values with this calculator. If you are looking to add or subtract some time from a date, please use the date calculator. Please provide the values in the fields above the "=". Then click the "Calculate" button to calculate. When entering, you can leave some fields blank. These fields will be taken as 0 by default.

Reference
Like numbers, time can be added or subtracted. However due to the definitions, time can not be simply added or subtracted like normal decimal numbers. The following are the definitions of the common time units.
Unit  Definition 
millennium  1,000 years 
century  100 years 
decade  10 years 
year (average)  365.242 days or 12 months 
common year  365 days or 12 months 
leap year  366 days or 12 months 
quarter  3 months 
month  2831 days Jan., Mar., May, Jul., Aug. Oct., Dec.—31 days Apr., Jun., Sep., Nov.—30 days. Feb.—28 days for common year and 29 days for leap year 
week  7 days 
day  24 hours or 1,440 minutes or 86,400 seconds 
hour  60 minutes or 3,600 seconds 
minute  60 seconds 
second  base unit 
millisecond  10^{3} second 
microsecond  10^{6} second 
nanosecond  10^{9} second 
picosecond  10^{12} second 
How We Measure Time
We all know that there are sixty seconds in a minute, and sixty minutes in an hour, but why do we measure time in these terms?
We measure time in units of sixty, because we still use the system worked out by the Sumerian civilization, which flourished in Mesopotamia about 2,000 B.C.E.
The Sumerians invented the Sexagesimal System based on the number 60. Sixty seconds in a minute, sixty minutes in an hour  and the Sumerians also had a calendar with 360 (60x6) days in a year. To make it work for all units of time, the Sumerians also fixed twelve hours (double six) in a day and twelve at night, and roughly 12 months in a year (especially in a 360 day year).
The Babylonians, who lived after the Sumerians in Mesopotamia, also based their mathematics on the number 60. This was because the number 60 is a superior highly composite number, having factors of 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60 (including those that are themselves composite), facilitating calculations with fractions.
The Egyptians also based time calculation on the number 60. They divided their year into three 120day seasons of four months of 30 days. They defined the hour as either 1/12 of daytime or 1/12 of nighttime.
The Greeks had a fairly complex calendar: A year lasted twelve months, sometimes with a thirteenth month added in. The extra month usually came after the sixth month. The beginning of the year fell in the summer.
Other Greek regions started their year at different times — Sparta, Macedonia in fall, Delos in winter.
Later Greek astronomers, including Hipparchus and Ptolemy, defined the hour as 1/24 of an average solar day. The Hellenistic year was divided into synodic months, meaning the average lunar month. It is the average interval in days between exact conjunctions of the Moon and the Sun (as observed from the Earth). The current value of the synodic month (rounded to five decimal places) is 29.53059 days.
The Roman civil day was the same one familiar to us, a 24hour day beginning at midnight. The early Romans differed from other ancient societies in their choice of midnight to mark the start of the day: Greeks marked the day beginning from sunset, Babylonians from sunrise.
By the early 3rd Century B.C.E., Roman practice further divided the day into four equal parts. Mane and ante meridiem, morning and forenoon, came before the passing of midday. Afternoon and evening followed midday.
In the middle of the 2nd Century B.C., the Romans adopted a type of timekeeping device known as the clepsydra. This device was also copied from examples used by the Greeks. The clepsydra, or waterclock, is simply a container holding water with several openings at the bottom to allow the water to escape at a regulated rate.
The simple clepsydra later came into general use in households to keep time, and was especially useful because they could divide the day into periods of equal measure, i.e. into hours. However, even these devices had certain problems in use; water, for example, flowed out of the holes faster or slower depending on the temperature of the water. Ctesibius of Alexandria is credited with perfecting the clepsydra around 250 B.C.E. His device was based on maintaining a constant pressure of water in the main tank to provide a regulated flow at a constant speed through the outlet into another container. This device worked year round, and could be outfitted with a small figurine holding a pointer that moved along a graded measure to indicate the hours.
During the Middle Ages, a combination of water clocks, sun dials, and candle clocks were used to tell time though none of those could tell time to the minute. While the best water clocks could tell time to the quarter hour, it wasn't until the wideuse and improvement of mechanical clocks that people were able to tell time to the minute.
Jost Burgieven is credited with inventing the minute hand in 1577. The minute hand wasn't widely added to clocks until the 1680s. The mechanical clock was developed to a level of reasonable accuracy in the 14th and 15th centuries.
In the 17th century, pendulum clocks were developed, and this enabled measurement of seconds as well as minutes. The Royal Society in the U.K. first proposed the second as a unit of time. The duration of a beat or half period (one swing, not back and forth) of a pendulum one meter in length on the earth's surface is approximately one second.
Nevertheless, with the development of pendulum clocks keeping mean time (as opposed to the apparent time displayed by sundials), the second became measurable. The second pendulum was proposed as a unit of length as early as 1660 by the Royal Society of London. The duration of a beat or half period (one swing, not back and forth) of a pendulum one meter in length on the earth's surface is approximately one second.