Add Hours Calculator

Created by Georgi Georgiev
Last updated: Sep 05, 2026

1. How to use the calculator
2. Adding and subtracting hours and minutes
• Add hours to time
• Subtract hours from time
3. Crossing time zones and DST

    How to use the calculator

First, choose whether you will be adding hours and minutes, or subtracting hours and minutes from a time. The "Initial time" field of the add hours calculator shows the current local time of the browser when loaded. There is the option to click "set to now" to update it to current time if you've kept it open for a while. The time is in 12-hour or 24-hour format, depending on your browser locale settings.

Fill in the hours and minutes fields, as necessary. You can always use just one, for example, enter 5 to add 5 hours and leave the minutes field empty. Equivalently, you can enter 300 in the minutes field and leave the hours input empty (300 minutes equals 5 hours). To add 2 hours and 30 minutes to a time it is easiest to enter "2" in the hours field and "30" in the minutes field.

After clicking the button to perform the calculation, the tools shows you the resulting time. If that time is a day or more in the future this will be noted, likewise for crossing into a different day when subtracting hours. Find examples below.

    Adding and subtracting hours and minutes

In some cases you may want to do hour calculations by hand. The section below explains the basics of hours and minutes math, and also demonstrates how this add hours calculator works.

Add hours to time

Say now is 11:40 AM and you want to calculate what time it will be in three and a half hours (3.5 h) which is the same as the addition of 3 hours and 30 minutes. You can follow this simple algorithm in which h is the number of hours and m is the number of minutes to add, in this case h = 3 and m = 60 · 0.5 = 30.

1. Add the minutes m to the minutes of the initial time.
2. Divide by 60 with remainder. Add the quotient to the number of hours to add h. The remainder is the resulting minutes.
3. Add the number of hours h to the initial hour.
4. Divide by 24 with remainder. The quotient is the number of days ahead, while the remainder is the resulting hour.
5. Write down the resulting hours and minutes and note the number of days ahead, if greater than zero.

The initial time is assumed to be in 24-hour format. You can use the 24-hour to 12-hour converter for the conversion or to get an explanation on how to do it yourself.

In the above example:

1. 30 + 40 = 70 minutes
2. 70 modulo 60 results in a quotient of 1 and a remainder of 10 minutes. We add 1 to the 3 hours to add, to get 4.
3. 4 + 11 = 15 hours
4. 15 / 24 results in a quotient of 0 and a remainder of 15.
5. The resulting time is 15:10 or 3:10 PM in a 12-hour format.

The answer: it will be 15:10 (3:10 PM) in 3 hours and 30 minutes.

Subtract hours from time

Subtraction of hours and minutes works similarly to how adding hours works. If now is 17:15 (5:15 PM) and you want to know what time was two and a half hours ago (2 hours and 30 minutes ago), use the following approach:

1. Subtract the minutes m to the minutes of the initial 24-hour formatted time.
2. If the result is negative, divide its absolute value by 60 with remainder. Add the quotient plus one to the number of hours to subtract h. 60 minus the remainder gives you the resulting minutes.
3. Subtract the number of hours h from the initial hour.
4. If the result is negative, divide by 24 with remainder. The quotient is the number of days behind the initial time, while 24 minus the remainder is the resulting hour.
5. Write down the resulting hours and minutes and note the number of days behind, if greater than zero.

The example works out to: 15 - 30 = -15. 15 modulo 60 results in a quotient of 0 and a remainder of 15. 60 - 15 = 45 minutes. The hours to subtract become 2 + 1 = 3. 17 - 3 = 14 hours. The number of hours is positive so the resulting time is 14:45 or 2:45 PM, meaning it was 14:45 2.5 hours ago.

Unlike simpler approaches, the above algorithms for adding hours and minutes and for subtracting will work even if the hours to add / subtract are greater than 24 and also if the minutes to add or subtract are greater than 60. Therefore the tool can be used to calculate what time was 72 hours ago, 48 hours ago, 24 hours ago and so on. The above hour calculations can be confirmed by checking the number of hours between times or dates using our hours calculator.

    Crossing time zones and DST

A major complication when you add hours (or subtract hours) is present if you are taking a flight or otherwise moving between time zones, or when a Daylight Savings Time (DST) change takes effect where you are. The change in time zone offset needs to be factored in the above calculations, adding or subtracting the hours and minutes difference between the initial time zone offset and the final one.

An alternative would be to perform the basic hours math by hand or by using this add hours calculator, then simply convert the resulting time to the new time zone using our time zone converter. This way you can correctly estimate at what time you will arrive at your destination and whether you can make it for a meeting or not.