Add Days Calculator

Deadlines, follow-ups, maturity dates and retrospectives all ask the same question: what date is N days from today? This calculator adds (or subtracts) a whole number of days to a start date and returns the resulting date, its weekday, and the number of days applied. It handles month-end rollovers, leap years and year boundaries automatically.

Enter values

Results

Resulting date: 2024-09-14
Day of the week: Saturday
Days added: 30

How is the Add Days calculated?

Enter the start day, month, year and the number of days to add (negative to go backwards). The calculator builds the start date in UTC, adds the offset in whole days, and returns the new date and its weekday. No time-of-day component is involved — only calendar dates. Month-end rollovers, leap years and year boundaries are handled for you: one day after 31 March is 1 April, and one day after 31 December is 1 January of the next year.

Formula

Result date = start date + N days (UTC). If N is negative, the date moves backward. Weekday = Gregorian weekday of result date. Month lengths and leap days are handled by the UTC date arithmetic.

Example

Start 15 August 2024, add 30 days: result = 14 September 2024 (Saturday). Start 20 October 2024, add 100 days: result = 28 January 2025 (Tuesday). Start 1 March 2024, subtract 1 day: result = 29 February 2024 (Thursday, leap year). Start 31 December 2024, add 1 day: result = 1 January 2025 (Wednesday).

Things to remember

Use positive numbers for deadlines ('30 days from invoice date') and negative for look-backs ('90 days before expiry'). The weekday helps you see if the result lands on a weekend — if it does, combine with the working-days calculator to find the next business day.

Frequently Asked Questions

Can I subtract days to go into the past?

Yes — enter a negative number for 'days to add'. The calculator will move backwards correctly across month and year boundaries.

Does it handle 29 February correctly?

Yes. Adding 1 day to 28 February 2024 gives 29 February 2024. Adding 1 day to 28 February 2023 gives 1 March 2023. Subtracting 1 day from 1 March 2024 gives 29 February 2024.

What is the maximum number of days I can add?

The input accepts up to ±36,500 days (about 100 years). For longer spans, use the date-difference calculator in reverse.

Why does the result show a weekday?

Many deadlines that land on a weekend need to move to the next business day. The weekday lets you spot this immediately and use the working-days calculator to adjust.