Quick answer
Use calendar-month arithmetic with a stated end-of-month rule. Converting months into a fixed number of days drifts immediately.
What 'add a month' means
The operation people intend is: keep the day number, advance the month, roll the year if needed. One month after 15 March is 15 April; six months after 15 March is 15 September. The interval in days varies — 31 here, 184 there — and nobody minds, because the question was calendar-shaped.
Adding 30 days instead gives 14 April and 11 September for those two cases. Close, wrong, and wrong by a growing amount. Over a year of monthly steps the two methods separate by about five days.
Use this guide with the right tool: Open the Add Months to Date Calculator for a calendar date shifted forward by complete months. If the question shifts, compare it with the Subtract Months From Date Calculator or the Date Difference Calculator.
The short-month problem
One month after 31 January cannot be 31 February. Something must give, and there are two standard answers. Clamping moves to the last day of the target month: 28 or 29 February. Rolling overflows into the next month: 3 March in a common year.
Clamping is the more widely used convention and is what the calculators here do. Rolling appears in some financial and scheduling systems. Neither is wrong, but they diverge by up to three days, and a schedule built on both cannot be reconstructed.
Clamping is not reversible
This is the subtle consequence. Add one month to 31 January with clamping and you get 28 February. Subtract one month from 28 February and you get 28 January — not the 31st you started from. The operation loses information.
For a sequence of monthly dates this matters a great deal. Stepping month by month from 31 January, a naive implementation clamps to 28 February and then continues from the 28th, producing 28 March, 28 April and so on. The correct approach keeps the original day number as the anchor and applies it fresh each month, giving 31 March, 30 April, 31 May.
Where the choice has consequences
Subscription renewals, medication schedules, contractual milestones, notice periods expressed in months, and probation end dates all use calendar-month arithmetic, and all involve dates near month ends often enough for the rule to matter.
Where a contract or policy specifies the treatment, follow it. Where nothing does, clamp, be consistent, and record the convention alongside any generated schedule so it can be checked later.
Worked example
Work through it with your own dates and the difference will usually be obvious. If it is not, the convention in use is the first thing to check.
Common mistakes to avoid
- Adding 30 days per month. Correct in four months of the year. The error accumulates to about five days annually.
- Stepping from each clamped result. Anchor on the original day number each time, or a monthly schedule migrates permanently to an earlier day.
- Assuming the operation is reversible. Adding then subtracting a month does not always return the original date. Clamping loses the day number.
Use the right calculator
The tools below apply this method directly. Where a result depends on a convention, the page says which one it used.
Frequently asked questions
Which rule does this site use?
Clamping to the last day of the target month, with the original day number preserved as the anchor for sequences.
What does a contract usually mean by 'one month'?
Calendar month, ending on the corresponding day of the following month. Where that day does not exist, jurisdictions differ — check the governing law.
Is there a reversible alternative?
Not one that also behaves sensibly. Storing the original day number alongside the generated date is the practical answer.