Add Days to Date Calculator

Date & TimeLast updated: August 17, 2026

Adding a period of time to a date comes up constantly: payment terms, return windows, the end of a trial, a course of medication, a visa period, or a project deadline. Because months have different lengths and leap years get in the way, working out something like 'what date is 90 days from now' by hand is easy to get wrong.

This calculator adds days, weeks, months, or years to a start date, or counts the same period backwards. The result is shown in long date format together with the day of the week, and you also see how many days the period works out to.

Today's date is used by default.
How much time to add or subtract (a whole number).

Formula

Days/Weeks: Result = Date ± (Amount × 1 or 7) days
Months: month number ± Amount; the day number stays, rolling into the next month if it overflows
Years: year ± Amount; February 29 shifts to March 1 in a non-leap year

The most common mistake when adding months is treating every month as 30 days. This calculator uses calendar months: adding 1 month to January 31 lands on March 2 or 3, because February has no 31st. For contracts that mean '1 month = 30 days', choose days as the unit instead.

How to Calculate

  1. Pick the start date — it defaults to today.
  2. Enter the amount to add or subtract as a whole number (e.g. 90).
  3. Choose the unit: days, weeks, months, or years.
  4. Choose the direction: 'Add' to move forward, 'Subtract' to move back.
  5. Read the resulting date, the day of the week, and the difference in days in the results section.

Worked Examples

90 days later (payment due date)

An invoice dated Monday, August 17, 2026 with 90-day terms falls due on Sunday, November 15, 2026. Because the due date lands on a weekend, payment usually slips to the following business day.

Resulting date: Sunday, November 15, 2026 · Day of the week: Sunday · ISO format: 2026-11-15

Six months later (contract renewal)

A six-month contract starting on August 17, 2026 ends on Wednesday, February 17, 2027. Since calendar months are used, the actual span is 184 days; a rough estimate that treats every month as thirty days is misleading.

Resulting date: Wednesday, February 17, 2027 · Day of the week: Wednesday · ISO format: 2027-02-17

Going back 45 days

Counting 45 days back from August 17, 2026 lands on Friday, July 3, 2026. This is how you plan preparation time backwards from an application deadline.

Resulting date: Friday, July 3, 2026 · Day of the week: Friday · ISO format: 2026-07-03

Frequently Asked Questions

What date is 90 days from now?
It depends on the start date: the calculator adds exactly 90 calendar days to the date you pick and shows the result together with the day of the week. For example, 90 days after August 17, 2026 is Sunday, November 15, 2026.
Why is the result different from what I expected when adding months?
Months are added on the calendar: the day number is kept and the month changes. If the target month has no such day (January 31 plus 1 month, for instance), the date rolls into the next month and you get March 2 or 3. For fixed 30-day periods, choose days as the unit and enter multiples of 30.
Can I add business days?
This calculator adds calendar days; it does not skip weekends. If you need '10 business days from now', add roughly 14 calendar days and then check how the span splits into business days with the Days Between Two Dates calculator.
Can I calculate a date in the past?
Yes. Choose 'Subtract' in the operation field and the calculator counts the period backwards. For example, you can go back 45 days from a delivery date to find the day work has to start.
Are leap years taken into account?
Yes, the calculation runs on the real calendar. Adding 1 year to February 29, 2024 gives March 1, 2025, because 2025 has no February 29; when you add days, the 366-day leap years are counted automatically.
What is the ISO format in the results for?
The YYYY-MM-DD layout (2026-11-15, for example) is the international standard; it transfers into spreadsheets, databases, and official forms without ambiguity, and it sorts chronologically by default.