SumDate Calculator
Count the days between two dates, or add and subtract days from any date — perfect for deadlines, countdowns, due dates and planning ahead.
Duration
How It Works
The "days between two dates" mode counts the calendar days from the start date up to, but not including, the end date — the standard way durations are measured. If you tick the inclusive option, both the start and end dates are counted, which adds exactly one day. That matches how most people count a vacation ("we are away Friday through Sunday — three days") or a rental period. The result is then broken down into weeks (days ÷ 7) and approximate months (days ÷ 30.44, the average length of a month), so you can see the same span in the unit that makes most sense.
The "add / subtract days" mode moves forward or backward from a starting date by the number of days you enter, landing on a new calendar date. It handles month and year boundaries automatically, including leap years — adding days across February 29 works exactly as the calendar does. This is handy for deadlines ("30 days from today"), due dates, follow-up reminders and expiry dates.
Worked examples: from January 1, 2026 to November 12, 2026 is 315 days — exactly 45 weeks, or about 10.3 months. Counted inclusively (both end dates included) it is 316 days. And adding 30 days to November 12, 2026 lands on Saturday, December 12, 2026. You can check the first example by adding up the months: January has 31 days, February 28 (2026 is not a leap year), March 31, April 30, May 31, June 30, July 31, August 31, September 30, October 31, plus 11 days into November — 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 11 = 315.
Frequently Asked Questions
What is the difference between inclusive and exclusive counting?
Exclusive counting (the default) measures the gap between two dates: January 1 to January 2 is 1 day. Inclusive counting includes both end dates, so January 1 to January 2 is 2 days. Use inclusive counting for trips, bookings and events that span whole days.
Does the calculator handle leap years?
Yes. All calculations use real calendar dates, so February 29 is counted in leap years (such as 2024 and 2028) and skipped in other years. Adding days across a leap day lands on the correct date.
Does it count business days or weekends?
It counts calendar days, including weekends and holidays. If you need business days only, a good rough guide is to take about 5/7 of the calendar-day total, then adjust for public holidays.
Why does it say the end date must be after the start date?
In "days between" mode the end date has to be on or after the start date. If your dates are the other way round, just swap them — the duration is the same either way.
How far ahead can I calculate?
You can add or subtract up to 36,500 days (about 100 years) in one calculation, and the "days between" mode works for any two dates your browser's date picker supports.
Last updated: November 2026