Dates and time
Add or subtract days from a date
Find the date 90 days from now, 6 weeks ago, or 18 months out. Month math keeps the same day of the month when it exists.
What this means
- Adding 90 days gives February 7, 2027, a Sunday.
- When a month has no matching day, such as January 31 plus one month, the result moves to the last day of that month.
What if
About this calculator
What this calculator does
It moves a date forward or back by a number of days, weeks, months, or years and reports the new date and weekday.
How it is calculated
days and weeks: move by that many 24-hour calendar days
months and years: change the month, keep the day, and clamp to the month's last day if needed
How to read the result
The result date is the calendar date you land on. The start day itself is day zero.
Sources and method
Calendar math on the Gregorian calendar with dates only, so time zones and daylight saving do not apply. Unit tests cover leap years, month ends, and February 29. Adding months clamps to the last day of the month.
Questions
Is 90 days the same as 3 months?
No. Three calendar months can be 89 to 92 days. Pick the unit your rule uses.
What is January 31 plus one month?
February 28, or February 29 in a leap year.
What happens to February 29?
Adding months or years to a day that does not exist in the target month moves it back to the last day of that month. February 29 plus one year is February 28. The age calculator counts a February 29 birthday on March 1 in other years, because a birthday is not reached until the day after February 28.
Does it skip weekends?
No. Use the business days calculator for weekdays.
Does the start day count as day 1?
No. The start date is day zero, so adding 1 day gives the next day.