Time Calculator

Created by Georgi Georgiev
Last updated: Sep 05, 2026

1. How to do time calculations
2. What time will it be in X hours or minutes
3. Examples of time addition and subtraction
4. Commonly used time units
5. History of time keeping

    How to do time calculations

To add time or subtract time with the time calculator you need to know the initial time you want to start with. For example 15 Sept. 2026, 12:00 (British) or Sep 15, 2026, 11am (American). The tool allows you to add or to subtract from that time, returning the final result with accuracy to the second, if needed.

When calculating time it is important to be aware that there are exceptions to some common assumptions. For example, a year is generally accepted to be 365 days but sometimes it is 366. A month is around four weeks but it is neither 28 days, nor the 30.5 days sometimes used for simplicity. Accurate calculations require exact starting date and time, and exact time counts, not approximations. This online clock calculator takes no such shortcuts and delivers accurate and precise results, regardless of whether the time period involves a leap year or not, for example.

If performing time addition or time subtraction by hand, a good approach is to first convert the input into a convenient unit with a fixed number of seconds (see the table). Then begin 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 it be a given number of hours or minutes into the future. It makes questions like "what time is 30 minutes from now" and "what time will it be in 30 minutes" easy to answer.

Upon loading the page of our add time calculator the "Initial time" field shows your current date and time. Proceed to enter the number of hours or minutes in the respective field and press "Calculate". For example, enter 10 in the "Hours" field to add 10 hours to the current time and see what time it will be in 10 hours. Enter "30" in the "Minutes" field to see what time it will be in 30 minutes.

You can also specify any initial time you want, For example, you can enter 9:30 and then "30" in the minutes field to see what is 30 minutes from 9:30 (it is exactly 10:00 o'clock). Or enter 12:45 as initial time and then 45 minutes to make the time calculator tell you what is 45 minutes from 12:45 (it's 13:30 or 1:30 PM). Try our dedicated tools: add hours calculator, hours from now calculator, and the add days calculator to see if they are a better fit for your use case.

    Examples of time addition and subtraction

Example 1: Say you arrange for a meeting 30 minutes from now, assuming now is 1:45 PM. What time will it be in 30 minutes? What time is your meeting at? Basically, the question is what is 30 minutes from 1:45 PM?

To find the answer, first add 30 to 45, resulting in 75. Since an hour has just 60 minutes, we need to subtract 60 from 75, which leaves a remainder of 15. So we need to add one hour to 13 (1 PM), resulting in 14 o'clock or 2 PM. Adding the 15 minutes left as a remainder gives the final answer: 14:15. So 30 minutes after 13:45 is 14:15 (2:15 PM).

Example 2: You might wish to cook a meal which needs to be in the oven for 6 and a half 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 3: 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.

* About every four years there is a "leap year" with 366 days due to February having one more day. For example, 2024, 2028, and 2032 are leap 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

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.