Quick answer
A calendar month advances the month field. When the target month lacks the original day number, the result depends on the rule your app or schedule uses.
Step by step
- Find the target year and month first. Check whether it has the starting day number.
- If the day exists, keep it. If it does not, decide whether the rule uses the last valid day or carries the extra days into the next month.
- For a recurring schedule, decide whether each occurrence is calculated from the original date or from the previous occurrence. Record the rule.
Worked examples
Clamp to the last valid day
Jan 31, 2025 plus one calendar month lands on Feb 28 under a last-valid-day rule. In 2024 it lands on Feb 29.
Carry extra days forward
Under an overflow rule, Feb 31, 2025 carries three days past Feb 28 and becomes Mar 3.
Fixed days
Jan 31, 2025 plus 30 days lands on Mar 2.
What to check
The Add Days to a Date calculator handles a fixed signed number of calendar days. Use it when you know the day count, and check the schedule’s rule for calendar months.
Related guides
Sources
See the calculation methodology or report an error.