Month-end arithmetic is ambiguous
What is one month after 31 January? There is no 31 February, so a choice must be made. This calculator clamps to the last valid day, giving 28 or 29 February — the same convention used by most calendar software and by contract law in many jurisdictions.
Years and months are applied before days, so "add 1 month and 5 days" to 31 January gives 5 March in a common year (28 February, then five days).
Common uses for date arithmetic
Contract terms, notice periods, warranty expiry, visa validity, prescription refills and project milestones all reduce to adding a period to a known date. Getting it right matters because the difference between "30 days" and "one month" can be a day or three either way.
Where a legal deadline is involved, check whether the period is expressed in calendar days or business days. The working-days figure shown above lets you compare both readings before committing to one.
The order of operations matters
This calculator applies years first, then months, then days — the convention used by most date libraries and by contract drafting. Adding one month and one day to 31 January gives 1 March in a common year: the month lands on 28 February, then the day is added.
Reversing the order would give a different answer. If precision matters, express the period in a single unit — days are unambiguous where months are not.
Frequently asked questions
Does adding 30 days give the same result as adding 1 month? +
Only for months with 30 days. Adding a month keeps the day-of-month where possible; adding 30 days is a fixed count that drifts through the calendar.
What date is 90 days from today? +
Set the days field to 90 and leave the others at zero. The result also shows the weekday, which is useful when the deadline needs to fall on a working day.
Why does subtracting a month from 31 March give 28 February? +
There is no 31 February, so the result clamps to the last valid day of the target month. This is the standard convention across calendar software.