Months Between Dates Calculator

The months between dates calculator tells you instantly how many complete calendar months separate any two dates — along with a full breakdown into years, days, and weeks. Whether you need to verify a notice period, check how long a subscription has been active, calculate a child’s age in months, or measure an investment horizon, this tool gives you the answer in one click.

How it works

The tool counts complete calendar months — the standard used by HR systems, tenancy agreements, bank statements, and most legal contracts. The algorithm is straightforward:

1. Compute the raw month span: (endYear − startYear) × 12 + (endMonth − startMonth).
2. If the end day-of-month has not yet reached the start day-of-month, subtract 1 — that final month is incomplete.
3. Derive years as floor(totalMonths / 12) and remaining months as totalMonths mod 12.
4. Compute total days from the raw millisecond difference (ms / 86 400 000, rounded).
5. Derive whole weeks as floor(totalDays / 7) and leftover days as the remainder.

This approach handles month-length irregularities (28, 29, 30, or 31 days) and leap years automatically because it works on the calendar position of each date, not on a fixed day-length approximation.

Worked example

Say you want to know how long a fixed-term tenancy ran:

• Start date: 15 September 2023
• End date: 28 February 2025

Step 1 — raw months: (2025 − 2023) × 12 + (2 − 9) = 24 − 7 = 17. Step 2 — end day (28) is greater than start day (15), so no subtraction. Step 3 — result: 17 complete months (1 year 5 months). Total days: 532. Whole weeks: 76.

If the tenancy had ended on 10 February 2025 instead: end day (10) < start day (15), so we subtract 1 → 16 complete months.

Formula note

The core formula in plain text:

months = (endYear - startYear) * 12 + (endMonth - startMonth)
if endDay < startDay: months = months - 1

This is equivalent to finding how many times you can advance a date by exactly one calendar month without overshooting the end date — the standard “floor” definition of elapsed months used across finance, HR, and law.

Common use cases

• Employment and notice periods — most UK contracts specify notice in whole months; this confirms whether the minimum has been served.
• Lease and tenancy terms — verify a 6-month, 12-month, or 24-month period exactly.
• Subscription billing — know precisely how many billing cycles have elapsed.
• Age in months — useful for child development milestones and paediatric references.
• Loan and mortgage terms — cross-check the number of repayments remaining.
• Project timelines — measure a sprint cadence or milestone gap.

The quick-preset buttons (Last 3 mo, Last 6 mo, Last 1 yr, Last 2 yr, Last 5 yr) fill both date fields automatically so you can benchmark common spans without typing.

Every calculation runs entirely in your browser — nothing is sent to any server.