Date difference calculator
The gap between two dates seems straightforward until you need precision. The Apollo 11 mission lasted exactly 8 days, 3 hours, 18 minutes, and 35 seconds — but "8 days" appears in most history books because human memory rounds. Contract law counts every day: a 30-day return window that starts June 1 ends June 30, not July 1, and that single day has decided countless lawsuits.
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Business days require additional calculation. Calendar days include weekends and holidays; business days don't. A 30-calendar-day period contains roughly 21-22 business days (30 × 5/7), minus any holidays. Legal deadlines often specify which type: "30 days" vs. "30 business days" vs. "30 court days." Misreading this distinction has cost plaintiffs their cases.
Inclusive vs. exclusive counting matters. Hotels count nights (check in Jan 1, out Jan 3 = 2 nights). Age counts completed days (born Jan 1, today Jan 3 = 2 days old). Prison sentences vary by jurisdiction — some count the intake day, some don't. Our calculator uses exclusive counting (start date not included), matching ISO 8601 conventions.
Historical date math gets complicated before 1582. The Gregorian calendar replaced the Julian calendar in October 1582 — but not everywhere at once. Britain didn't switch until 1752, Russia until 1918. Calculating days between "July 4, 1700" and "July 4, 1800" requires knowing which calendar each date used. Our calculator assumes Gregorian throughout.