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How to Use the Date Calculator

The Date Calculator adds or subtracts days, weeks, months and years from a given date.

Keyboard with red button with about written on it.

©bigstockphoto.com/jurgenfr

If you wish to exclude non-working days or business days from your location, please use our Business Date Calculator.

How many days between 2 dates?

Setting Up a Calculation

To make a calculation, please make your selections in the set-up form and click the green “Calculate new date” button.

Choosing the Start Date

Start by entering the date you wish to use as a starting point for your calculation. You can use any date in the years 1 through 3999. Type the numbers in the boxes, use the drop-down menus that open if you click inside the boxes, or use the calendar pop-up window. To use the current date, please click on the “Today” link.

Adding or Subtracting Days

In the “Add/Subtract” drop-down menu, select if you wish to add or subtract days. Then enter the number of days, weeks, months, and years into the appropriate fields on the right side.

Adding Hours, Minutes, and Seconds

If you would like to calculate an exact moment in time using the time of day and adding or subtracting hours, minutes, and seconds, please click on the “Include the time” link. Then enter the time of day on the start date into the fields on the left; and the number of hours, minutes, and seconds to be added or subtracted into the fields on the right.

Using the Repeat Option

It is also possible to add or subtract the same amount of days multiple times. Simply enable the “Repeat” option, then enter the number of times the calculation is to be executed.

Example: To find out the dates 11, 22, 33, and 44 days from the start date, enter 11 into the “Days” field, enable the “Repeat” option and enter 4 into the “Calculate ... times” field.

Reading the Results

Once you click the green button, the calculation results are displayed on the same page, below the set-up form.

The calendar below the yellow box shows an overview of the time frame used in the calculation. It includes the start date (blue) and the end date (yellow).

The Calculator's Logic

Because the Gregorian calendar's months have varying lengths, adding or subtracting months is a little trickier than one might think, and in some cases, there are several possible results.

When adding or subtracting months, the Date Calculator keeps day numbers intact wherever possible. So, if you add 1 month to January 10, the result is February 10. Similarly, adding 1 month to February 10 results in March 10. The differing month lengths are ignored in this case.

However, this is not possible when subtracting 1 month from one of the last days in a month. An example: Subtract 1 month from May 31. The result in our calculator is April 30 because both dates represent the last day of the month. However, if you add 1 month to April 30, the result is May 30.

In general, the Date Calculator deals with the larger time units before the smaller time units. For example, years are added before months, weeks, and days. In some cases, the results would differ if we added the small units first.

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