What this date calculator does
Add or subtract days, weeks, months or years from a date, or measure the difference between two dates in days, weeks, months and years. Useful for deadlines, invoice terms, retention windows and contract end dates.
Counting dates is not simple arithmetic
Months have different lengths, which makes month arithmetic ambiguous: adding one month to 31 January has no obvious answer, since 31 February does not exist. This calculator clamps to the last valid day of the target month, which is the same convention most databases use. When a calculation is contractual, check which convention applies: counting the start day, the end day, or business days excluding weekends.
Frequently asked questions
Is the start date counted in the difference?
The difference is the elapsed time between the two dates, so the start date is not counted as a full day. For contracts that say N days from a date, check whether the convention includes the start day.
How does adding one month to 31 January work?
There is no 31 February, so the result clamps to the last valid day of the target month, 28 or 29 February. This matches the behaviour of most databases and is the safest convention.
Does it count business days?
The calculator works in calendar days. Business day counts depend on which weekends and public holidays a country observes, so those should be checked against the relevant calendar rather than assumed.
Is it affected by my time zone?
Date arithmetic is performed on calendar dates rather than instants, so a date does not shift because of your time zone. Convert instants to local dates first if your input came from a timestamp.