About the Date Calculator
Two everyday date problems, one tool. First: how far apart are two dates — for deadlines, lease terms, subscription periods, project timelines, or a countdown to an event? Second: what date lands a given number of days, weeks, months, or years before or after a known date — for notice periods, return windows, warranty expirations, or medication schedules?
Both modes work on the real calendar: months keep their true lengths, leap days appear exactly where the leap-year rules put them, and every result names the weekday, because “30 days from now” is more useful when you know it’s a Sunday.
Measuring from a date of birth instead? The Age Calculator.
How Days Between Dates Are Counted
The duration mode reports the same gap three ways — total days, weeks plus leftover days, and calendar years/months/days:
Total days = End date − Start date (end date not counted) Weeks = Total days ÷ 7, remainder shown as days Years / months / days = calendar breakdown, borrowing from real month lengths
Worked example: January 1, 2026 to July 10, 2026 is 190 days — 27 weeks and 1 day, or 0 years, 6 months, 9 days. The end date isn’t counted, so a Friday-to-Friday week is 7 days, not 8. If your use case counts both endpoints (days of a rental including pickup and return day), add 1 to the result.
Adding & Subtracting Dates
Days and weeks are exact arithmetic: 30 days after July 10, 2026 is Sunday, August 9, 2026, and 2 weeks before July 10, 2026 is Friday, June 26, 2026. Months and years move to the same day number in the target month — and when that day doesn’t exist, the result clamps to the month’s last day: January 31, 2026 + 1 month = February 28, 2026, and February 29, 2024 + 1 year = February 28, 2025.
That clamping rule is the convention used by banks for monthly billing dates and by spreadsheet EDATE functions, so results here line up with statement dates and contract anniversaries. If you need “last day of each month” behavior instead, start from the 1st of the following month and subtract a day.
Days in Each Month
Month lengths are why date math needs a calculator — a quick reference:
| Month | Days | Month | Days |
|---|---|---|---|
| January | 31 | July | 31 |
| February | 28 (29 in leap years) | August | 31 |
| March | 31 | September | 30 |
| April | 30 | October | 31 |
| May | 31 | November | 30 |
| June | 30 | December | 31 |
Leap years: divisible by 4, except century years that aren’t divisible by 400 — 2024 and 2028 are leap years, 2026 isn’t; 2000 was, 1900 wasn’t. A common year has 365 days, a leap year 366.
Calendar Days vs Business Days
This tool counts calendar days — every day including weekends and holidays, which is what leases, interest, visas, and most legal deadlines use. Business-day counting excludes weekends and public holidays, which differ by country and even by state or province. For a quick estimate, every full week contains 5 business days (190 calendar days ≈ 27 weeks ≈ 135 business days, before subtracting holidays); for anything contractual, check the specific holiday calendar that applies.