Planning a project deadline, counting down to an event, or figuring out what date falls exactly 90 days from today? The add days to date calculator gives you the answer in under a second — no mental arithmetic, no paper calendar flipping. Enter any start date, type the number of days you want to add (or subtract), and the tool instantly shows the result in every format you might need: human-readable long date, ISO 8601, US format, UK format, day of week, ISO week number, quarter, day of year, days remaining in the year, and Unix epoch timestamp.
How it works
Date arithmetic sounds simple — “add 30 days” — but edge cases trip up manual calculation constantly. What is 31 January + 1 day? What is 28 February + 1 day in a leap year? The calculator avoids these traps by working with the JavaScript Date object’s setDate method: it reads the day-of-month, adds your offset, and lets the date normalisation engine handle all month-boundary roll-overs and leap-year logic automatically.
The formula in plain terms is:
result = setDate(getDate() + N)
where N is the signed integer you enter. Negative N subtracts days. The result is then formatted into multiple representations so you can copy whichever format your spreadsheet, database, or colleague expects.
ISO week number follows ISO 8601: week 1 is the week containing the first Thursday of the year, and weeks run Monday through Sunday. The implementation finds the start of week 1 by locating the nearest Monday on or before 4 January, then counts complete 7-day intervals from that anchor.
Day of year is the 1-based count of days elapsed since 1 January (so 1 January = day 1, 31 December = day 365 or 366). Days remaining in the year is simply the year length (365 or 366 for leap years) minus the day of year.
Unix timestamp is the number of whole seconds since 1970-01-01T00:00:00Z. The calculator derives it from the local-midnight representation of the result date.
Worked example
Suppose today is 1 June 2026 and you need to know what date is 90 days from now.
- Start date:
2026-06-01 - Days to add:
90 - Result: 30 August 2026 (Sunday)
Additional details the calculator surfaces:
| Field | Value |
|---|---|
| ISO 8601 | 2026-08-30 |
| US format | 08/30/2026 |
| UK format | 30/08/2026 |
| Day of week | Sunday |
| Relative to today | 90 days from today |
| ISO week | Week 35 of 2026 |
| Quarter | Q3 2026 |
| Day of year | 242nd day of 2026 |
| Days remaining in year | 123 days |
| Unix timestamp | 1756512000 |
Now try subtracting: enter -30 to go back 30 days from 1 June 2026, and you get 2 May 2026 (Saturday), week 18, Q2.
Formula note
There is no single equation for “date plus N days” because the Gregorian calendar has variable month lengths. The correct approach is to use calendar arithmetic — adjust the day component and let the normalisation rules handle overflow. Any formula that converts to a day count (e.g. Julian Day Number) and adds N before converting back is mathematically equivalent. The calculator uses the browser’s native date engine, which correctly handles all Gregorian edge cases including the February 29 leap-day boundary and the year-end roll-over from 31 December to 1 January.