OneFreePlace · Tool #123

Months between dates—without pretending “a month” has one definition.

Compare calendar months, completed same-day months, end-of-month-aware months, average Gregorian month equivalents and raw month slots. Inspect exact days, proration differences, monthly anniversaries and what happens when a date such as the 31st meets a shorter month.

✓ Multiple month definitions✓ End-of-month intelligence ✓ Exact days + calendar remainder✓ Add/subtract months ✓ Anniversary CSV

Month Difference Calculator

Compare definitions · reverse months · inspect schedules

Date span

Choose two dates.

Start date

End date

Primary answerAll methods stay visible on the right.

Calendar span adds whole months first using end-of-month clamping, then counts the leftover days.

Monthly anniversary policyUsed by the planner and add-months tool.

When the exact day does not exist

Clamp example: February 28 + 1 month = March 28. End-of-month mode keeps February 28 → March 31 when February 28 is the starting month’s last day.

Add / subtract monthsIndependent reverse calculation.

Base date

Months to add

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Month difference

Forward span

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calendar months

Choose two dates.

Calendar span—clamped whole months + leftover days

Strict completed months—requires reaching the same day number

EOM-aware completed months—month-end ↔ month-end can count as full

Average Gregorian months—exact days ÷ 30.436875

Raw month slots crossed—year/month index difference; ignores day

Exact elapsed days——

Why the answers can differ

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Proration lab

See how a monthly amount changes depending on the month definition. This is arithmetic only—not a claim about a contract, payroll, rent or billing standard.

Monthly amount (any currency / units)

Calendar fraction——

Average Gregorian—days ÷ 30.436875 × amount

30-day convention—days ÷ 30 × amount

Interpretation guide

Calendar spanAdd as many calendar months as possible from the earlier date using clamping, then count the remaining calendar days.

Strict completed monthsCounts a month only after the later date reaches the original day number. Jan 31 → Feb 28 is therefore 0.

EOM-aware completed monthsIf both endpoints are month-end, a shortened month can still complete the month. Jan 31 → Feb 28 can therefore be 1.

Average Gregorian monthsUses 365.2425 ÷ 12 = 30.436875 days per average month. Useful as a continuous approximation, not a calendar-month count.

Monthly anniversary planner

Generate repeat dates from a base date using your selected anniversary policy. Useful for billing cycles, subscriptions, project checkpoints and recurring reviews.

Base date

Months

Offset	Date	Weekday	Days since prior	Month-end?

Month-edge diagnostics

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Earlier date—

Later date—

Calendar fractional months—

Signed direction—

Why this matters

A “month difference” used for tenure, subscriptions, billing, service periods or age-like calculations may need a calendar definition. Analytics may instead need a continuous numeric month equivalent. Reporting systems may count month buckets regardless of day.

This calculator exposes those choices rather than producing one unexplained decimal and calling it universally exact.

Month calculations are convention-sensitive.Before using a result for a contract, invoice, legal deadline, payroll rule or official eligibility calculation, use the exact convention required by that rule. The comparison here helps you identify the assumption; it does not choose the governing policy for you.