Months Calculator

Calendar month arithmetic advances dates by whole months while respecting each month's actual length. Adding one month to January 31 does not produce February 31 (which does not exist) — it produces February 28 in common years or February 29 in leap years, using "end-of-month clamping." Adding one month to January 30 yields February 28/29 for the same reason. Adding one month to March 15 yields April 15 — straightforward when the day exists in the target month.

Example: start January 31, 2026 and add 1 month. January 31 + 1 month = February 28, 2026 (2026 is not a leap year). Add another month: March 31, 2026 — because March has 31 days, the day-of-month restores to 31. This "last day" behavior matches financial systems, subscription renewals, and Excel's EDATE function. Adding 12 months to February 29, 2024 (leap year) yields February 28, 2025.

Counting months between dates yields full calendar months plus leftover days. From March 15, 2024 to June 11, 2026: 2 years (to March 15, 2026) plus 2 months and 27 days (to June 11) = 26 months and 27 days, or 2 years 2 months 27 days. This differs from dividing total days by 30.44 — calendar months align with how humans discuss durations ("two and a half years").

Financial applications include loan terms (36-month auto loan), lease periods (12-month rental), and subscription billing (monthly renewal on the signup date). Pregnancy tracking uses weeks primarily but months provide intuitive milestones: 3 months ≈ 13 weeks (first trimester), 9 months ≈ 40 weeks (full term, though calendar months are closer to 10 lunar months).

Add or subtract a number of months from a start date, or enter two dates to count the months between them. The calculator handles end-of-month clamping, leap-year February, and year rollover automatically — no manual day-counting required.