ADD, SUBTRACT OR CONVERT MONTHS

Use date mode to add or subtract calendar months with end-of-month clamping. Use conversion mode to translate a number of months into years and approximate weeks or days.

Starting date

Direction

Months

How to use this calculator

1Step 1Choose a mode: add/subtract months, or convert months ↔ years.

2Step 2Enter your date or month count.

3Step 3Read the result — with an edge-case spotlight when month clamping happens.

What the result includes

Dual mode: date math + conversion
Month-length clamping explained live
Jan 31 + 1mo = Feb 28 spotlight
Months ↔ years/weeks/days table
Calendar-tab mode switcher

Why month math breaks naive calculators 🗓️

Months are the only calendar unit with variable length — 28, 29, 30 or 31 days depending on the month and the year. So "add one month" has no single arithmetic answer. The convention (used by courts, banks, and this tool) is same-day-next-month with clamping: January 31 + 1 month becomes February 28 (or 29 in leap years), because February 31 doesn't exist. August 30 + 6 months = February 30 → clamped to February 28/29. This calculator detects every clamp and tells you it happened.

Conversion mode: the average month 🔁

For pure unit conversion there's no specific calendar, so we use the standard average: 1 month = 30.44 days (365.25 ÷ 12, accounting for leap years). That gives you approximate weeks, days and years — appropriate for approximate unit comparisons such as “how long is 18 months?”, while date mode gives exact answers for real dates.

Conversion reference

Months	Years	Days ≈
6	0.5	182.6
12	1	365.25
18	1.5	547.9
24	2	730.5

Real-world month math 📚

Subscriptions & trials: "3-month trial from Jan 31" ends April 30, not May 1.
Contracts: notice periods in months need the clamping rule, not 30-day math.
Age in months: babies are counted in calendar months — the Baby Age Calculator uses the same engine.
Loan tenures: 60-month loans — convert the term to years in conversion mode.

Years work the same way with their own leap-day traps — see the Years Calculator. And for mixing days into the equation, the Days Calculator has your back. 🎯

🗓️ FAQ

What is January 31 plus 1 month?

February 28 normally, February 29 in leap years — the date clamps to the target month's last day when needed.

How many days are in X months?

Convert mode uses the 30.44-day average month (365.25 ÷ 12) for estimates; date mode uses exact calendar math.

Can I subtract months?

Yes — the same clamping applies backwards (March 31 − 1 month → Feb 28/29).

Why not multiply by 30?

Months are 28–31 days long; ×30 drifts days per year. Real calendar math avoids the drift.

What is 3 months after January 31?

February 28 (or 29 in leap years) — months have different lengths, so the calculator clamps to the last valid day and explains it in the spotlight box.

How many years is 60 months?

Exactly 5 years; the conversion mode also shows weeks and days using the 30.44-day average month.

How do I add months to a date?

Mode A: enter the date and month count, choose add or subtract, and get the calendar-correct result date.