Understanding the result
The result is the calendar date reached after moving the starting date by your chosen amount, along with its weekday. The extra figures show the result in ISO format and how many days it lies from the starting date.
If a month or year step lands on a day that does not exist, a note explains that the result was moved to the last day of that month.
What does this calculator do?
It answers questions like "what date is 90 days from now?", "what was the date 6 weeks ago?" or "when does a 12-month term starting on 31 March end?".
Days and weeks are exact lengths. Months and years follow the calendar, which makes them a little more subtle.
How it works
Days and weeks: the starting date is converted to a day number, the amount (weeks are multiplied by 7) is added or subtracted, and the result is converted back to a date. This handles month lengths, leap years and year boundaries exactly.
Months and years: the month is moved by the amount and the day of the month is kept. If the target month is shorter, the result is the last day of that month. So 31 January plus 1 month is 28 February (29 in a leap year), and 29 February 2024 plus 1 year is 28 February 2025. The result never rolls forward into the following month.
Subtracting works the same way in reverse, so 31 March minus 1 month is 28 February in a common year.
Examples
15 January 2026 plus 45 days
15 January plus 16 days is 31 January, plus 28 more is 28 February, plus one more is 1 March. The result is 1 March 2026.
31 January 2026 plus 1 month
There is no 31 February, so the result is 28 February 2026, the last day of the month. The tool flags this so you can see the date was adjusted.
29 February 2024 plus 1 year
2025 is not a leap year, so the result is 28 February 2025. Adding 4 years instead lands on 29 February 2028.
Things to keep in mind
- Adding 1 month and then another 1 month can give a different answer from adding 2 months at once, because each step is clamped to the month length. Add the total in one step when you can.
- Contracts and legal deadlines sometimes define month arithmetic differently, for example "the corresponding day of the next month, or the last day if there is none", which matches this tool, or "30 days" regardless of the month. Follow the definition that applies to you.
- The tool works with whole dates. It does not add hours or take time zones into account.
- The result must fall between the years 1 and 9999.