Time Calculator
This calculator can be used to "add" or "subtract" two time values (Day, Hour, Minute, Second). Input fields can be left blank, which will be taken as 0 by default.
Add or Subtract Time from a Date
Use this calculator to add or subtract time (days, hours, minutes, seconds) from a starting time and date. The result will be the new time and date based on the subtracted or added period of time. To calculate the amount of time (days, hours, minutes, seconds) between times on two different dates, use the Time Duration Calculator.
The Nature of Time and Its Measurement
Like other numbers, time can be added or subtracted. However, due to how time is defined, differences exist in how calculations must be computed when compared to standard decimal numbers.
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Standard Units of Time
The following table shows some of the common units used in time measurement, from millennia down to picoseconds:
[Image of Standard Units of Time Measurement Table]
| 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
|
| 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 (SI unit) |
| millisecond | $10^{-3}$ second |
| microsecond | $10^{-6}$ second |
| nanosecond | $10^{-9}$ second |
| picosecond | $10^{-12}$ second |
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Concepts of Time: A Philosophical and Scientific History
Ancient Greece: Aristotle (384–322 BC)
The ancient Greek philosopher **Aristotle** defined time as “a number of movement in respect of the before and after.” His view essentially required the existence of some kind of **motion or change** for time to be measured. He believed time was **infinite and continuous** and was one of the first to question how time can exist when it is composed only of two different kinds of non-existence (the past and the future).
Newton & Leibniz: Absolute vs. Relational Time
A major controversy over the nature of time began with the opposing views of **Sir Isaac Newton** and **Gottfried Leibniz**.
- Newton’s Absolute Time: Newton defined absolute time as a “duration” that exists and flows uniformly without any regard for external factors. This absolute time, also called **Newtonian time**, could only be understood mathematically because it is imperceptible. Relative time, conversely, is what humans perceive—a measurement of duration based on the motion of objects like the sun and moon.
- Leibniz’s Relational Time: Contrary to Newton, Leibniz argued that time only makes sense in the presence of objects with which it can interact. He saw time as a concept, similar to space and numbers, that allows humans to **compare and sequence events**. This argument, known as **relational time**, posits that time itself cannot be measured; it is only the subjective way humans perceive and sequence events.
The Bucket Argument
The “bucket argument,” or **Newton’s bucket**, arose from the debate between Newton’s spokesman Samuel Clarke and Leibniz. Newton used the example of spinning water in a bucket, which forms a concave surface, even if the bucket itself is stopped. Since this concavity wasn’t based on the interaction between the water and the bucket, Newton claimed the water was rotating relative to a third entity: **absolute space**. He argued this was necessary to explain rotation and acceleration where a relationalist view fell short. This Newtonian concept remained prevalent for almost two centuries.
Einstein: Spacetime and Relativity
The theory of relativity, compiled and described by **Albert Einstein**, profoundly transformed the understanding of time. Unlike Newton, Einstein, building on Leibniz’s view of relativity, introduced the idea of **spacetime** as a connected concept.
- Einstein posited that the **speed of light ($c$) in a vacuum is constant** for all observers, independent of the light source’s motion.
- To maintain the invariance of the speed of light, measurements of both **space and time must change** for observers in different inertial frames of reference (moving at different relative velocities).
- Time Dilation: The simplest way to state this is: if an object moves **faster through space**, it will move **slower through time**, and vice versa. Theoretically, time would stop if an object could reach the speed of light.
It is noteworthy that Einstein’s **theory of general relativity** finally answered Newton’s bucket argument after nearly two centuries. General relativity states that an object moving against a **geodesic** (the generalization of a straight line in curved spacetime) experiences a force. The water in the bucket is concave because it is rotating with respect to a geodesic, not “absolute space.”
The evolution of these theories proves that even the most established concepts can be overturned. Despite advances in quantum physics, **time is still not fully understood**. It may only be a matter of time before our understanding evolves again!
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How We Measure Time: The Calendar and the Clock
Today, time is typically measured using two distinct forms: the **calendar** and the **clock**. Both are based on the **sexagesimal numeral system**, which uses 60 as its base. This system originated in ancient Sumer during the 3rd millennium BC and was adopted by the Babylonians.
- Why Base 60? The number 60 is a **superior highly composite number**, having 12 factors (1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60). This simplifies many fractions involving sexagesimal numbers. For instance, 1 hour (60 minutes) can be evenly divided into 30, 20, 15, 12, 10, 6, 5, 4, 3, 2, and 1 minute.
Development of the Second, Minute, and 24-Hour Day
The concept of dividing the day into smaller, equal parts has a long history:
- Egyptian Divisions: The Egyptian civilization is credited with first dividing the day using **sundials**, splitting the time between sunrise and sunset into 12 parts. They used 12 selected stars to create 12 divisions of the night, leading to the concept of a **24-hour day**. However, these “hours” varied in length with the season.
- Equinoctial Hours: Around 147–127 BC, the Greek astronomer **Hipparchus** proposed dividing the day into 12 hours of daylight and 12 hours of darkness based on the equinoxes. These **equinoctial hours** resulted in days with hours of equal, fixed length, although this concept only became common with the advent of mechanical clocks in the 14th century.
- Minute and Second: Hipparchus developed a system of longitude encompassing 360 degrees. **Claudius Ptolemy** later subdivided each degree into 60 parts, and each of those into 60 smaller parts, which became known as the **minute** and **second**, respectively.
- The Gregorian Calendar: While many calendar systems existed, the **Gregorian calendar**, introduced by Pope Gregory XIII in 1582, is the most commonly used worldwide. It was an improvement upon the Julian calendar (proposed by Julius Caesar in 45 BC) which was inaccurate, causing astronomical events to advance by approximately 11 minutes per year.
Early Timekeeping Devices
Early devices were used primarily to regulate work or religious practices, not necessarily to tell the time of day:
- Oil Lamps and Candle Clocks: Used to mark the passage of time from one event to another.
- Water Clocks (Clepsydra): Arguably the most accurate clock of the ancient world. They functioned based on the regulated flow of water from or into a container, with the measured water determining the passage of time.
- Hourglasses (Sandglasses): First appeared in the 14th century and were initially used like candle clocks. Later, they were calibrated by more accurate clocks to measure specific time periods.
- Pendulum Clocks: The first mechanical clock regulated by a mechanism with a “natural” period of oscillation was created by **Christiaan Huygens** in 1656. He refined it to have errors of fewer than 10 seconds a day.
- Atomic Clocks: Today, these are the most accurate time measurement devices. They use electronic oscillators based on **cesium atomic resonance**. The **second**, the SI unit of time, is now calibrated based on measuring periods of the radiation of a cesium atom.