Days Between Dates: Why It's Harder Than You Think
How many days between March 1 and March 1 next year? 365 — unless it's a leap year. Date arithmetic gets complex fast with 28–31 day months and inclusive vs exclusive counting. Days Between Dates: Why It's Harder Than You Think How many days between March 1 and March 1 next year? 365 — unless it's a leap year. Date arithmetic gets complex fast with 28–31 day months and inclusive vs exclusive counting. 01. The leap year trap Leap year if ÷4, except ÷100 (not leap), except ÷400 (leap). 2000 was leap, 1900 was not, 2100 won't be. Multi-year spans: can't multiply by 365. Jan 1 2024 → Jan 1 2028 = 1,461 days (3×365 + 366), not 1,460. 02. Inclusive vs exclusive counting 'Days until event': exclude today, include target. 'Trip days' (Jan 5–8): include both = 4 days. The ±1 matters for contracts and billing. Most calculators (ours included) use exclusive-start: Jan 1 → Jan 2 = 1 day. 03. Months in differences 'X years, Y months, Z days' is ambiguous — months vary 28–31 days. Is 'one month after Jan 30' Feb 28 or Mar 2? We show both total days (unambiguous) and approximate breakdown. Takeaway: Date math is deceptively complex. For deadlines or contracts, use a calculator — leap years and month lengths will trip you up.