Time Calculator
With this tool you can either sum or deduct two time entries. If you leave an input empty it will be assumed to be zero.
Add or Subtract Time from a Date
Enter a base date‑time and specify a span of days, hours, minutes or seconds to increase or decrease it. The calculator returns the adjusted date‑time. For finding the interval between two distinct dates, switch to the Time Duration Calculator.
Time Calculator in Expression
This utility lets you combine several time quantities via a simple expression. Write each amount followed by d, h, m or s (days, hours, minutes, seconds) and separate them with + or -. For instance, "1d 2h 3m 4s + 4h 5s - 2030s" is a correctly formed input.
Time behaves like a numeric quantity that can be summed or deducted, but its structure leads to calculation rules that differ from ordinary decimal math. Below is a table listing typical 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 | 28-31 days Jan., Mar., May, Jul., Aug. Oct., Dec.—31 days Apr., Jun., Sep., Nov.—30 days. Feb.—28 days for a common year and 29 days for a 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 |
Concepts of Time:
Ancient Greece
Throughout history philosophers and scientists have offered many definitions of time. Aristotle (384‑322 BC) described it as “the number of motions with respect to before and after,” essentially tying time to change and movement. He regarded time as endless and continuous, believing the universe has always existed and will continue to do so. Aristotle also hinted that time’s existence depends on the coexistence of two opposite kinds of non‑existence, a idea that sparked later debates, especially those sparked by Newton and Leibniz.
Newton & Leibniz
In his *Philosophiae Naturalis Principia Mathematica*, Newton treated space and time as absolute entities. He claimed that absolute time flows uniformly, independent of any external influence, a notion he labeled “duration.” Because this absolute time cannot be sensed, it is only accessible through mathematical description. In contrast, the time we experience — relative time — is measured by observing the motion of celestial bodies like the Sun and Moon. This perspective is commonly called Newtonian time.
Leibniz rejected Newton’s absolute time, arguing that time only makes sense when there are objects to relate to. For him, time is a mental construct, similar to space or numbers, that lets us order and compare events. In this relational view, time itself is not directly measurable; it merely reflects how we subjectively arrange happenings throughout our lives.
The so‑called bucket argument, stemming from the exchange between Newton’s advocate Samuel Clarke and Leibniz, illustrates this clash. A bucket filled with water hangs stationary; when the bucket and water are spun, the water’s surface becomes concave. If the rotation stops, the surface stays concave for a while. Since the shape change cannot be explained by interaction between bucket and water alone, Newton inferred that the water rotates relative to a third entity—absolute space. He maintained that such an immutable backdrop is needed when relational explanations fall short, and this Newtonian picture dominated physics for almost two centuries.
Einstein
A host of researchers—including Mach, Michelson, Lorentz and Poincaré—paved the way for modern physics, but it was Albert Einstein who assembled and articulated the theory of relativity and the Lorentz transformation. Contrary to Newton’s belief that time ticks identically for all observers, Einstein, extending Leibniz’s relational insight, merged space and time into a single spacetime fabric. He asserted that the speed of light in vacuum, c, is constant for every observer regardless of the source’s motion, linking spatial and temporal measurements. Consequently, observers moving at different velocities experience altered distances and time intervals. A classic illustration involves a spaceship traveling near light speed: to an external observer, the ship’s clocks run slower, and at light speed they would effectively stop.
In simple terms, the faster something moves through space, the slower it progresses through time, and vice versa—an effect required to keep the speed of light unchanged.
It’s interesting that after almost two hundred years, Einstein’s general relativity finally answered Newton’s famous bucket problem. In GR an inertial reference frame is defined as one that travels along a spacetime geodesic – the curved‑space analogue of a straight line. The theory tells us that any body that veers off a geodesic feels a force, while a body in free‑fall experiences no force because it stays on the geodesic. On Earth, however, the planet’s surface pushes the object away from its natural geodesic, which we interpret as a gravitational force. Consequently, the concave surface of water in the rotating bucket is explained by rotation with respect to a geodesic rather than by any "absolute space" or distant stars as Mach suggested.
Across the ages, countless theories about the nature of time have risen and fallen, showing that even the most elegant models can be overturned. Though quantum mechanics and many other fields have deepened our knowledge, a complete picture of time remains elusive. Perhaps one day the constancy of light’s speed, a cornerstone of Einstein’s work, will be revised, opening the door to true time‑travel.
How we measure time:
Today we mainly rely on two systems to keep track of time: calendars and clocks. Both stem from the sexagesimal (base‑60) counting system, a heritage of ancient Sumerian mathematics that the Babylonians later adopted. Sixty is a highly composite number with twelve divisors, which makes it handy for dividing hours into equal parts. For instance, a single hour (60 minutes) can be split neatly into 30, 20, 15, 12, 10, 6, 5, 4, 3, 2 or 1‑minute intervals, a convenience that helped the system endure.
Development of the second, minute, and concept of a 24-hour day:
The first culture known to segment the day into smaller units was the Egyptians, as evidenced by their early sundials that broke daylight into twelve sections. Since sundials ceased to work after sunset, Egyptian astronomers turned to the night sky, assigning twelve stars to create a complementary twelve‑part division of darkness. This dual twelve‑part scheme likely inspired the modern 24‑hour day. Their hour lengths, however, changed with the seasons, making summer hours longer than winter ones. It wasn’t until around 147‑127 BC that the Greek astronomer Hipparchus proposed equal daylight and night hours based on the equinox, establishing the 24‑hour system that later became standard with the advent of mechanical clocks in the 14th century.
Hipparchus also introduced a framework of longitude lines that spanned 360 degrees. Later, Claudius Ptolemy refined this grid, dividing each degree into 60 arc‑minutes and each minute into 60 arc‑seconds, a hierarchy still used in modern navigation.
Among the many calendars created throughout history, the Gregorian calendar is the one most of the world uses today. Pope Gregory XIII instituted it in 1582, building on the earlier Julian calendar devised by Julius Caesar in 45 BC. The Julian system drifted by about eleven minutes each year, causing the dates of equinoxes and solstices to shift. The Gregorian reform corrected this error, bringing the calendar back in line with the solar year. For a deeper dive into its development, check the date calculator.
Early timekeeping devices:
Early time‑keeping devices varied widely across cultures, primarily serving to divide day or night into work or ritual intervals rather than to tell the exact hour. Examples include oil lamps and candle clocks, which marked the passage of time by the consumption of fuel. The clepsydra, or water clock, was probably the most precise ancient instrument, measuring time by the regulated flow of water. In the 14th century, hourglasses appeared, initially serving the same purpose as lamps and candles. As mechanical clocks grew more accurate, they were used to calibrate hourglasses for specific time spans.
Christiaan Huygens built the first pendulum clock in 1656, introducing a mechanism whose natural swing period provided a reliable time base. He refined the design to achieve errors under ten seconds per day. Today, atomic clocks have eclipsed all earlier technologies in precision. They employ an electronic oscillator tuned to the resonance frequency of cesium atoms; the cesium standard defines the second, the SI unit of time, by counting the cycles of this atomic radiation. While other atomic species are used in specialized clocks, cesium remains the most common and accurate.