How adding months works
Months have 28, 29, 30 or 31 days, so “one month later” cannot be a fixed number of days. The calculator follows the usual calendar convention: it keeps the same day number and moves the month. If that day does not exist in the target month, it uses the last day of that month instead, and the result tells you when this has happened.
| Start date | Months | Result |
|---|---|---|
| March 15 | + 3 | June 15 |
| August 31 | + 1 | September 30 (month end) |
| January 30, 2027 | + 1 | February 28, 2027 (month end) |
| January 30, 2028 | + 1 | February 29, 2028 (leap year) |
| May 31 | − 3 | February 28/29 (month end) |
| October 5, 2026 | + 18 | April 5, 2028 |
Months vs. 30-day periods
Some agreements define a “month” as 30 days rather than a calendar month. These give different answers: 6 calendar months from January 1 is July 1 (181 days), while 6 × 30 days is June 30. If your contract or rule says “days”, use Add Days to Date instead. Note also that adding one month twice is not always the same as adding two months once: January 31 + 1 month = February 28, + 1 month = March 28, but January 31 + 2 months = March 31.
Uses for adding months
- Subscription renewals, rental terms and loan schedules.
- Warranty expiry dates and probation periods.
- Baby milestones such as 6 or 18 months — combine with the Birth Date Calculator.
To measure the months between two dates, use the Date to Date Calculator. Years can be added in the main Date Calculator.