Date Calculator (Add/Subtract)

The date calculator adds or subtracts a given number of days from a base date. Useful for contract expiry dates, end of medication, project deadlines, and more.

Enter a base date and the number of days to add or subtract (negative for a past date) to instantly see the result date.

For example, enter 100 for the date 100 days from today, or -30 for the date 30 days ago.

Date Calculator (Add/Subtract) Calculation Method

Result date = base date + entered days (negative = past date)

Base date

Days to add (negative allowed)

About This Calculator

A date calculator adds or subtracts days, weeks or months from a starting date and returns the resulting calendar date. It is useful for working out contract deadlines, project milestones, return windows and payment due dates.

Enter a base date and the amount to add or subtract to get the resulting date and the day of the week.

Adding days to a date is not plain arithmetic once months are involved, because month lengths differ: one month after 31 January is 28 or 29 February under the clamping rule. Ninety days after 1 January lands on 1 April in a leap year and 31 March otherwise. Business-day calculations must additionally skip weekends and public holidays.

Frequently Asked Questions

How do I count days excluding weekends?

You need working-day arithmetic: add the number of weekdays and skip Saturday and Sunday, then adjust for public holidays. Simple date addition counts calendar days including weekends.

What happens when I add a month to the 31st?

If the target month is shorter, most libraries clamp to the last day — 31 January plus one month becomes 28 or 29 February. This is the standard behaviour in business-day calculations.

Why does one month after 31 January give 28 February?

No February date can match day 31, so most libraries clamp to the last day of the month. Other systems roll over into March, which is why financial contracts state the convention explicitly.

How do I count working days?

Count weekdays and subtract the public holidays of the relevant jurisdiction. Korean holidays include a lunar calendar component, so a fixed list of Gregorian dates will be wrong in some years.

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Sources & last update

2026-10-19