How to use the date calculator
- Days between dates: pick the start and end. You get the total days, the same in weeks and days, in years, months and days, and the number of weekdays.
- Add or subtract: pick a date, a number and a unit (days, weeks, months or years) to find a deadline, a due date or a date in the past, with its day of the week.
How the counting works
Each date is turned into a whole day number, so subtracting two of them gives the exact number of days with no clock times involved; this is what keeps daylight-saving changes from adding or losing a day. Years, months and days are counted the calendar way: whole months first (from January 31, one month later is the end of February), then the days left over. Adding months or years follows the same end-of-month rule.
What we tested
On October 14, 2026 we ran the page’s own functions on 5,000 random date pairs and additions between 1900 and 2100 and compared them with Python’s datetime, dateutil (calendar months and month-end clamping) and NumPy’s business-day count (Monday to Friday). The first run caught a real bug: weekday counts were wrong for dates before 1970, because of how negative numbers behave in JavaScript’s remainder; fixed before release.
| Check | Matches |
|---|---|
| days | 5,000 of 5,000 |
| years/months/days | 5,000 of 5,000 |
| weekdays (Mon-Fri) | 5,000 of 5,000 |
| weekday | 5,000 of 5,000 |
| adding days/weeks/months/years | 5,000 of 5,000 |
Invalid dates such as February 29, 2023 or April 31 are rejected, and leap days are accepted. In Chromium: with the browser on New York time, March 10 to 11, 2024 (a 23-hour day) counted 1 day and November 3 to 4 (25 hours) 1 day; January 31 to March 1, 2024 gave 30 days = 1 month, 1 day, 22 weekdays; January 31, 2024 plus one month gave Thursday, February 29, 2024; and the page fitted a 390 px phone screen with no errors
Frequently Asked Questions
Built by Derek Giordano · Part of Ultimate Design Tools