Use this add time / subtract time calculator to easily add or subtract time to any date, add/subtract hours, minutes or seconds to a particular time. Versatile online time adder to see what time will it be in X hours, minutes, days, etc.

Quick navigation:

- How to do time calculations
- What time will it be in X hours or minutes
- Examples of addition and subtraction of time intervals
- Commonly used time units
- History of time keeping and time calculation
- The importance of time measurement

## How to do time calculations

To add and subtract time with the time calculator you need to know the initial time you want to start with. For example 15 Sept. 2012, 12:00 (British) or Sept. 15, 2012, 11am (American). The clock calculator allows you to add or to subtract from that time, calculating the final result to the second, if needed.

When calculating time it is important to be aware that there are exceptions to some of the commonly accepted assertions. For example, a year is accepted to be 365 days but sometimes it is 366; a month is around four weeks but not precisely 28 days, etc. So, if relevant to your case, be sure to acknowledge this. This online time calculator, however, will be precise in its results regardless of whether you are working with a leap year or not, for example. Simply be sure to provide the correct data in the forms above.

If performing time addition or time subtraction by hand, a good approach is to first convert the input time interval into a convenient unit with a fixed number of seconds (see the table), and then to start with the greatest time unit that would be altered and move down towards the smallest one. This is most important when it comes to addition or subtraction of months, years, or decades.

## What time will it be in X hours or minutes

One of the most common applications of a time calculator is to determine what time will be a given number of hours into the future. Questions like "What time will it be in 20 minutes?" or "What time will it be in 20 hours?" can easily be answered by time algebra.

To get the answer while using our tool above, simply load the page (or reload if it is already open) so the "Initial Date & Time" fields show your current date and time. Then, enter the number of hours or minutes in the respective field and press "Calculate". For example, enter 10 in the "Hours" field to see what time it will be in 10 hours, or enter "10" in the "Minutes" field to see what time it will be in 10 minutes.

## Examples of addition and subtraction of time intervals

** Example 1:** You might wish to cook a meal which needs to be in the oven for 6.30 hours. Let's say right now it is 11:20 and you want to find out when to set the oven timer to. You need to enter "11:20" in the "Time" box and then using the "add time" function you will enter "6" in the "hours" box and "30" in the "minutes" box. By hitting the "calculate" button you will be shown the result in the bottom, which in this case is 17:50, or 5:50 pm.

** Example 2:** If your doctor asks you to go for a check-up in 18 days, you might want to set a reminder. To estimate when your check-up is due, first enter the current date, e.g. 20.05.2023 or May 20

^{th}, 2023. Then enter how many days you wish to add, in this case it is 18, and click "calculate" above. The time adder will do the rest.

## Commonly used time units

Understanding all time units is important for accurate time addition / subtraction. Converting all times, including the interval to add or subtract to seconds and then working with timestamps is often the preferred way of going about it.

Time unit | Equals | In seconds |
---|---|---|

Millennium / kiloyear | 1000 years | it depends |

Century | 100 years | it depends |

Decade | 10 years | it depends |

Year | 365 or 366 days* ~52 weeks | ~31,557,600 seconds |

Month | ~4 weeks 28-31 days | 2,419,200 - 2,678,400 seconds |

Week | 7 days 168 hours 10,080 min | 604,800 seconds |

Day | 24 hours 1,440 min | 86,400 seconds |

Hour | 60 minutes | 3,600 seconds |

Minute | 60 seconds | 60 seconds |

Second | base unit** | - |

* About every four years there is a "leap year" with 366 days due to February having one more day. For example, 2020, 2024, and 2028 are such years. This also affects the number of hours, minutes, and seconds in a year.

** The second is the SI (The International System of Units) base unit. It can be written as sec or s. From it all other units are derived.

## History of time keeping and time calculation

Understanding time keeping can help you better understand time calculations and their complexities. The system by which we keep time is called **Sexagesimal**and goes as far back as 2000 b.c. It is used for measuring time, angles, and geographic coordinates. It has **sixty as its base**: 60 minutes in an hour, 60 seconds in a minute. It is suggested that the reasons for the origins of the system are based on the fact that people are able to count to twelve on one hand (using the thumb). However, the civilizations first to invent this used either a 60-based or a 12-based system (duodecimal).

Egyptians are thought to be the first to invent the sundial and used it to divide the day into **twelve parts**. This was probably due to the number of lunar cycles or because of the counting technique using the thumb. At this point people did not think of the night as a continuation of the day, but rather the opposite of it, thus the number 12 instead of 24. It is unknown where the number 60 came from, however, it is possible that this was due to it being the smallest number divisible by the first six counting numbers (1-6) as well as by 10, 12, 15, 20, and 30.

It should be noted that the public did not think about minutes and seconds until much later. This happened only when the **mechanical clocks** were created to display minutes at about the end of the 16^{th} century. So until then, time calculations would mainly concern days, weeks, months, and years.

Until not so long ago seconds were defined **based on astronomical events**. However, in 1967 it was redefined as the duration of 9,192,631,770 energy transitions of the cesium atom. This led to **atomic timekeeping**, which is accurate to 1 second over 20 million years, and Coordinated Universal Time (UTC), allowing for much more precise time calculations such as those performed by our software.

## The importance of time measurement

The ability to measure time precisely is one of the greatest human inventions! Precise time measurement as well as addition and subtraction of time intervals are crucial to a variety of economic activities around the world and the importance of precise time-keeping cannot be underestimated.

Applications dependent upon it include communication systems where networks need to be synchronised precisely, power grids, and finances, where timestamps accompany each transaction you make, and GPS - the Global Positioning System satellite navigation systems, which rely on time stamped signals to provide us with a precise location of a GPS receiver. By measuring the signal from four (or more) satellites, the user's position can be determined. The time measurement in GPS has to be incredibly accurate since light travels 30 centimetres in one nanosecond so even a tiny error in the time measurement could put you off course by many meters or even miles.

Therefore time is used for the precision navigation of ships, airplanes, and family cars, but what GPS sattelites also do is broadcast timing signals from their atomic clocks. These are then used by the abovementioned industries. For example, wireless telephone and data networks use GPS atomic time and in-built time calculators to keep their base stations in perfect synchronization which allows for more efficient sharing of the limited radio spectrum by mobile phones. Similarly, digital broadcast radios make use of GPS time to ensure the bits from all radio stations arrive at receivers in lockstep, meaning listeners can tune between stations very quickly.

Another example is the U.S. Federal Aviation Administration (FAA), which uses GPS to synchronize reporting of hazardous weather from its 45 Terminal Doppler Weather Radars covering the United States. This is just one scenario in which distributed networks of instruments must work together to precisely measure common events. Such networks require timing sources that can guarantee time accuracy at several points.

In light of the above, while our **time calculator** is just a small tool that can help you in daily tasks that require time management, it is also a symbol of the progress of humanity. Use it to easily add or subtract time when needed.

#### References

1 Lombardi, M. (n.d.) "Why is a minute divided into 60 seconds, an hour into 60 minutes, yet there are only 24 hours in a day." [online] Available at: https://www.scientificamerican.com/article/experts-time-division-days-hours-minutes/

2 "Units of Measurement". [online] Available at: http://www.exactlywhatistime.com/measurement-of-time/units-of-measurement/

3 "GPS Timing" [online] Available at: https://www.gps.gov/applications/timing/