How to Use: Date Calculator

The Date Calculator allows you to calculate a date in the past or future by adding or subtracting any number of days, weeks, months, or years.

Getting Started with Your Calculation

  1. Enter the date you wish to use as a starting point for your calculation under Start Date.
  2. Under Add/Subtract, choose if you wish to add days or other time units to the date or if you wish to subtract them from it.
  3. Enter the number of Years, Months, Weeks, and Days to add or subtract.
  4. Click Calculate New Date.

Additional Options

Repeat the calculation

Enable Repeat and enter a number into the field that appears to add or subtract the same number of days or other time units multiple times.

Example: To find out the dates 11, 22, 33, and 44 days from the start date, enter 11 into the Days field, tick the Repeat box, and enter 4 into the field.

Include the time

The Include the time link takes you to the Date and Time Calculator where you can execute the same type of calculation, but including the time of day and the option to add hours, minutes, and seconds.

Include only certain weekdays

The Include only certain weekdays link takes you to the Business Date Calculator, which allows you to exclude public holidays or weekends from your calculation.

Frequently Asked Questions (FAQ)

Why is the result wrong?

The calculator adds or subtracts the larger time units before the smaller time units. This means that it adds years, then months, then days. Although the total number of days in January (31) and February (28) are different both are counted as 1 month. In some cases, this method can lead to unexpected results.

For example, if you add one month and one day to March 1, the result is April 2, as is to be expected (March 1 + 1 month = April 1; April 1 + 1 day = April 2).

However, if you add one month and one day to February 28, which in a common year is the day before March 1, the result is March 29, which is 4 days before April 2. The reason is that the calculator first adds 1 month to jump from February 28 to March 28, then it adds one day to arrive at March 29. If it added the days first, the result would be April 1 (February 28 + 1 day = March 1; March 1 + 1 month = April 1).

This may seem illogical at first, but the structure of the modern calendar means that there is no perfect way of calculating dates including months and years.

For calculations with several possible results, the calculator shows the Alternative result below the primary result.

How does the calculator treat differing month lengths?

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. 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.

Got unexpected results? See “Why is the result wrong?” above.

Does the calculator include leap years?

Yes, all our calculators account for leap years.

Why are the date fields in the wrong order?

You can change the order of the Day, Month, and Year fields by selecting a different Short Date format in My Units.

Can I edit my calculation?

To edit your calculation, scroll up to the top of the page, make your changes, and calculate again. The Make adjustment and calculate again is a shortcut to the top of the page.

Why does the result show the wrong days of the week?

This is most likely not an error but due to the fact that the Julian calendar was still in use in the year you entered. This calendar system was eventually replaced by today's Gregorian calendar, but the date this change happened differed from one country to another.

The calculator uses the Julian-Gregorian switch date of the country you select in the settings under My Units.

How does your algorithm work? Can you help me program my own?

We are a small team with a very extensive website to manage, so unfortunately, we do not have the capacity to share detailed information about our algorithms or provide programming help.

What kinds of date calculators do you offer?

  1. Calculating the duration between 2 points in time:
  2. Adding or subtracting days:

Where can I find more information about the site and its services?

The General FAQ Page answers your questions about timeanddate.com, our services, site-wide settings and customization options, as well as advertising and copyright policies.

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