Time Calculator

Add two lengths of time, such as laps in a race or tasks in a project, and the result appears in days, hours, minutes and seconds and as a total in each unit. For example, 2 hours 45 minutes plus 1 hour 30 minutes is 4 hours 15 minutes, or 4.25 hours. The tools below add time to a calendar date and evaluate longer expressions; the expression 1d 2h 3m 4s + 4h 5s − 2030s works out to 1 day 5 hours 29 minutes 19 seconds.

Concepts of Time:

Time is the ongoing sequence in which events happen, one after another, from the past through the present into the future. It is what clocks measure, and it lets us order events, compare how long they last and describe how fast things change. Physicists treat time as a dimension alongside the three of space, and Einstein's relativity showed that the passage of time is not absolute: it runs measurably slower for objects moving at high speed or sitting deeper in a gravitational field, an effect that GPS satellites must correct for every day.

In everyday life, time is a shared convention that lets people coordinate. Calendars divide it into years, months and days, and clocks divide days into hours, minutes and seconds.

How we measure time:

Humans first measured time with natural cycles: the day from the rotation of the Earth, the month from the phases of the Moon and the year from the Earth's orbit around the Sun. These cycles do not divide evenly into one another, which is why calendars need leap days and why lunar and solar calendars drift apart.

Today the base unit is the second, defined since 1967 by the vibration of cesium atoms: exactly 9,192,631,770 cycles of the radiation from a particular transition of the cesium-133 atom. Atomic clocks keep this time to within a second over millions of years. Because the Earth's rotation is slightly irregular, civil time (Coordinated Universal Time, UTC) has occasionally added a leap second to stay in step with the planet, though international timekeeping bodies have agreed to phase leap seconds out by 2035.

Development of the second, minute, and concept of a 24-hour day:

The 24-hour day comes from ancient Egypt, where astronomers divided the night into 12 parts using the rising of particular stars and the daylight into 12 parts with sundials. These hours originally changed length with the seasons; fixed, equal hours became standard only as mechanical clocks spread in the Middle Ages.

Dividing the hour into 60 minutes and the minute into 60 seconds traces back to the Babylonians, who counted in base 60, a system that divides evenly by 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30. Greek astronomers adopted base-60 fractions for their calculations, and the terms come from medieval Latin: the first small division (pars minuta prima) became the minute, and the second small division (pars minuta secunda) became the second.

Early timekeeping devices:

Sundials, among the oldest timekeepers, tell time from the shadow cast by the sun and were used in Egypt and Babylon thousands of years ago. They fail at night and on cloudy days, so people also used water clocks, or clepsydrae, which measure time by water flowing steadily into or out of a vessel. Candle clocks, incense clocks and hourglasses filled with sand measured fixed intervals.

Mechanical clocks driven by weights and an escapement appeared in European towers in the late 1200s and 1300s. Christiaan Huygens's pendulum clock of 1656 improved accuracy from about 15 minutes a day to under a minute, and later marine chronometers, notably John Harrison's in the 1700s, kept time well enough at sea to calculate longitude. Quartz clocks in the 20th century, and then atomic clocks, made precise time available to everyone.

Frequently asked questions