Adding time to a date
The date calculator finds the date that is a given number of years, months, weeks and days after, or before, a start date. Typical questions are “what is the date 90 days from today?”, “when is 6 weeks from now?” or “what was the date 18 months ago?”.
How each unit is added
- Days and weeks are exact: a week is always 7 days, and the calendar simply moves forward that many days, across months and years.
- Months and years follow the calendar: the day number stays the same in the target month. One month after March 15 is April 15, whatever the number of days in March.
- End of month: when the target month is too short, the date moves to its last day. January 31 plus one month is February 28, or February 29 in a leap year.
The calculator adds the years and months first, then the weeks and days.
Worked example
Starting on November 3, 2026 and adding 90 days:
- November has 27 days left after the 3rd, so 63 days remain after November 30.
- December uses 31 of them, leaving 32.
- January uses 31, leaving 1.
- Result: Monday, February 1, 2027, in ISO week 5.
Adding 3 months instead gives February 3, 2027: three calendar months are 92 days here, not 90.
Common deadlines
| Period | From November 3, 2026 |
|---|---|
| 30 days | December 3, 2026 |
| 60 days | January 2, 2027 |
| 90 days | February 1, 2027 |
| 6 weeks | December 15, 2026 |
| 6 months | May 3, 2027 |
| 1 year | November 3, 2027 |
Days or months?
Contracts, laws and policies use both, and they do not give the same date. A “30-day” return window and a “one-month” return window differ in months with 31 days or in February. When a deadline matters, check which unit the rule uses and whether the first day counts.
Counting backward
Choose Subtract to find a past date, for example the date 280 days before a due date or 18 months before a renewal. The same end-of-month rule applies: March 31 minus one month is February 28 or 29.