Months Between Two Dates

Counting months between two calendar dates is more subtle than it first appears because civil months have unequal lengths (28, 29, 30 or 31 days) and the modern Gregorian calendar inherits those quirks from the Julian reform of 46 BCE. Julius Caesar, advised by the Alexandrian astronomer Sosigenes, restructured the Roman calendar from a lunar to a solar basis, fixing month lengths to the irregular pattern we still use today. Pope Gregory XIII corrected the drift in 1582 by skipping ten days and refining the leap-year rule, but the unequal month lengths were left untouched.

The international standard ISO 8601 (first published in 1988, latest revision 2019) formalised the date format YYYY-MM-DD that this calculator accepts, eliminating regional ambiguity between American MM/DD/YYYY and European DD/MM/YYYY. ISO 8601 also defines period durations using the P prefix (e.g. P2Y3M for 2 years 3 months), which is how the result panel below labels output.

Months are used as the primary unit of accounting in mortgages (amortisation tables), payroll (salaried pay periods), subscription billing (SaaS, streaming), pregnancy (40-week / 9-month gestation), retirement vesting (typical 48-month cliff), and lease tenancy (12-month or 24-month terms). In each of these domains the question is rarely just whole months but a fraction of a month - hence this calculator reports a decimal month figure alongside the integer count.

There are two correct conventions for computing months between dates. The civil convention counts month anniversaries: 2026-01-15 to 2026-04-15 is exactly 3 months, with no partial. The actuarial convention treats every month as 30.4375 days (the average month length in a Gregorian year, 365.25 / 12) so the same span becomes 90 / 30.4375 = 2.957 months. This page returns both - whole months plus leftover days plus a decimal-month estimate computed the actuarial way.

Mortgage amortisation conventions, governed in the US by CFPB Regulation Z and TILA disclosure rules, almost always use the civil convention because the borrower pays on the same day of the month for the life of the loan. Insurance premiums and prorated subscription billing use the actuarial convention because they refund a partial period. SaaS pricing pages (Stripe, Chargebee, Recurly) all calculate proration this way.

The fiscal-month concept (where some companies define a custom 4-4-5 or 4-5-4 week structure) sits on top of the calendar-month abstraction this calculator works with. Retailers like Walmart and Target use the National Retail Federation 4-5-4 calendar, where each quarter consists of two 4-week months followed by one 5-week month, totalling exactly 52 weeks. If you need that, count weeks instead of months.

Demographically, the worldwide average lifespan of 73.3 years (UN World Population Prospects 2024 revision) translates to about 879 months. The average pregnancy lasts 9 months by the lay reckoning or 40 weeks (about 9.2 months) by the medical reckoning. The 90-day SaaS trial covers about 2.96 months. Knowing these reference points helps sanity-check the output of this tool.