Month Calculator
Add or subtract months from any date. Calculate resulting dates and months between two dates.
Add or Subtract Months
Months Between Dates
Month Arithmetic: The Complete Guide
Adding or subtracting months from a date seems simple until you encounter the calendar's fundamental irregularity: months have 28, 29, 30, or 31 days. What happens when you add one month to January 31? February doesn't have a 31st. What about subtracting 3 months from May 31? You land in February—a month with even fewer days. These edge cases make month arithmetic surprisingly complex, and our month calculator handles them all correctly using standard end-of-month clamping logic.
Adding N months: Move forward N months, keeping the same day number. If the target month has fewer days than the start day, clamp to the last day of the target month.
Example: January 31 + 1 month → February has 28 days → result: February 28
Example: January 31 + 2 months → March has 31 days → result: March 31
Worked Examples
Lease start: March 15, 2025
Lease term: 12 months
March 15 + 12 months = March 15, 2026
Day of week: Sunday
Subscribe on: January 31, 2025
Renewal: 1 month later
February has 28 days in 2025, so result = February 28, 2025
Next renewal (1 more month): March 28 or March 31 depending on billing system
Last menstrual period: June 10, 2025
Estimated due date: 9 months later
June 10 + 9 months = March 10, 2026
(Naegele's rule adds 7 days and subtracts 3 months for a more precise estimate)
Days Per Month Reference Table
The irregular pattern of days per month is the root cause of complexity in month arithmetic. The table below summarizes each month's length and highlights the months that cause the most end-of-month clamping issues.
| Month | Days | Quarter | Cumulative Days (End of Month) |
|---|---|---|---|
| January | 31 | Q1 | 31 |
| February | 28 / 29 | Q1 | 59 / 60 |
| March | 31 | Q1 | 90 / 91 |
| April | 30 | Q2 | 120 / 121 |
| May | 31 | Q2 | 151 / 152 |
| June | 30 | Q2 | 181 / 182 |
| July | 31 | Q3 | 212 / 213 |
| August | 31 | Q3 | 243 / 244 |
| September | 30 | Q3 | 273 / 274 |
| October | 31 | Q4 | 304 / 305 |
| November | 30 | Q4 | 334 / 335 |
| December | 31 | Q4 | 365 / 366 |
Use Cases for Month Calculations
Leases and Rental Agreements
Lease terms are almost always specified in months—6 months, 12 months, 24 months. Landlords and tenants need to know the exact end date to plan move-outs, renewals, and notice periods. A 12-month lease starting January 15 ends January 15 of the following year, but a lease starting January 31 may end January 31 or be clamped to the last day of shorter months during the term. Our calculator provides the precise end date for any start date and term length.
Subscriptions and Billing Cycles
Monthly subscriptions (streaming services, software licenses, gym memberships) typically renew on the same calendar day each month. When you subscribe on the 15th, you're billed on the 15th of each subsequent month. But subscribing on the 31st creates complexity—months with fewer than 31 days trigger end-of-month billing on the 28th, 29th, or 30th. Understanding these billing patterns helps you anticipate charges and budget accordingly.
Pregnancy Tracking
While doctors primarily track pregnancy in weeks (40 weeks from LMP), patients and families often think in months. "I'm 6 months pregnant" gives a more intuitive sense of progress. Adding 9 months from the last menstrual period gives an approximate due date, though Naegele's rule (subtract 3 months, add 7 days, add 1 year) is more precise. Our month calculator helps expectant parents convert between month-based and date-based pregnancy tracking.
Project and Contract Deadlines
Contracts and project plans frequently specify durations in months. "Deliverable due in 6 months" or "warranty valid for 24 months" are common phrases that require converting to a specific calendar date. A 6-month project starting February 15 ends August 15. Starting February 28 in a non-leap year, 6 months forward lands on August 28. Our calculator resolves these dates instantly, accounting for all calendar irregularities.
Our month calculator is free and runs entirely in your browser. For quick "X months from today" answers, try Months From Today. For full date math, use our Date Difference Calculator or Add/Subtract Days. For week calculations, see Weeks Between Dates.