What this calculator does
Find the exact date that falls a number of days, weeks, months or years from any date. Deadlines, notice periods, warranties and follow-up visits are often stated this way. Think "30 days from signing", "6 months from purchase" or "2 weeks before the event". This calculator turns that into a real calendar date and tells you the day of the week.
A table underneath lists common lookups from the same start date, so you don't have to type them. It covers 7, 14, 30, 45, 60, 90, 120 and 180 days, plus 3 months, 6 months and 1 year.
Days and weeks are exact, so adding them is plain counting. Months and years aren't a fixed number of days. So the calculator uses the rule most people expect: keep the same day of the month. If that day doesn't exist in the new month, it uses the month's last day instead.
Where it's used
Why this math matters outside the classroom:
- 90-day periods. Adding 90 days to January 1, 2026 lands on April 1, 2026.
- End-of-month dates. Adding one month to January 31, 2026 gives February 28, 2026, because February has no 31st.
- Warranties and returns. Shoppers add 30 days to a purchase date to know the last day an item can be returned.
- Library loans. Readers add 21 days to a checkout date to know when a borrowed book is due back.
How to use it
- Pick the start date. It starts out as today, in your own time zone.
- Choose whether to add (go forward) or subtract (go back).
- Enter any mix of years, months, weeks and days. Blank fields count as zero.
- Read the new date and its weekday. You'll also see how many days, and how many Monday-to-Friday weekdays, lie between it and the start. The table of common amounts sits below.
What the inputs mean
- Start date
- The date you count from. The start date itself is day zero, so adding 1 day gives the next day.
- Direction
- Add moves forward in time; subtract moves back. Every amount you enter goes in the same direction.
- Months
- Calendar months. Adding 1 month to the 15th gives the 15th of the next month, however long the month is.
- Weeks
- Each week is exactly 7 days, and it's combined with the days field.
Formula and method
total months = years × 12 + months intermediate date = start date ± total months (same day of month, clamped to the month's last day) total days = weeks × 7 + days result = intermediate date ± total days weekdays = Monday–Friday days from the earlier date up to, but not including, the later date
Years are turned into months (12 per year), and all the months are added first. That gives an in-between date, called the intermediate date in the formulas. The day of the month stays the same unless the new month is too short. Then the date moves back to that month's last day, which is called clamping. So January 31 plus one month becomes February 28, or February 29 in a leap year. Leap days work the same way: February 29 plus one year is February 28.
Next, weeks become days (7 per week) and join the days you entered. The calculator counts that many days forward or back on the calendar. It carries on across month and year ends, and it knows which years are leap years.
The order matters near the end of a month. In 2026, January 30 plus 1 month and 1 day gives February 28, then March 1, when months go first. Doing the day first would give January 31, then February 28. This calculator always adds months first, and the working shows each stage.
Worked example
A contract signed on Sunday, October 18, 2026 has a 30-day review period.
- No months or years are added, so the in-between date is still Oct 18, 2026.
- Add 30 days: the rest of October is 13 days (to Oct 31), and 17 more days reach Nov 17.
- Nov 17, 2026 falls 30 days after a Sunday. 30 = 4 × 7 + 2, so it is two weekdays later: a Tuesday.
- Weekdays: 4 full weeks hold 20, and the 2 extra days (Sunday and Monday) add 1, so 21 weekdays fall in the period.
- From the table, 90 days after the same start date is Saturday, Jan 16, 2027.
Result: The review period ends on Tuesday, Nov 17, 2026, which is 30 calendar days (21 weekdays) after the start date.
Understanding the result
The result is a calendar date with no time of day attached. So it doesn't shift with time zones or daylight saving time.
A rule that says "within 30 days" may or may not count the start date. This calculator treats the start date as day zero. If your rule counts it as day one, take one day off the amount.
If the working shows the date moved back to the end of a month, check that your rule expects that. Some agreements roll over into the next month instead.
Assumptions and limitations
- Dates use the Gregorian calendar, the one most of the world uses today. A leap year comes every 4 years, except century years that don't divide evenly by 400.
- Months and years are added before weeks and days.
- Every day counts, including weekends and holidays.
- Weekends and public holidays aren't skipped. To count working days only, use the business days calculator.
- Results are limited to the years 1 to 9999.
- Dates from before a country switched to the Gregorian calendar are worked out as if it had always been used.
Frequently asked questions
What is 90 days from today?
Leave the start date on today, choose Add, and enter 90 in the days field. Months have different lengths, so 90 days isn't the same as 3 months. The two can differ by a day or two.
Is 6 months the same as 180 days?
No. Six calendar months covers 181 to 184 days, depending on which months are involved. From October 18, 2026, 180 days lands on Friday, Apr 16, 2027. But 6 months lands on Sunday, Apr 18, 2027. Contracts and notices usually say which one they mean.
Why does adding one month to January 31 give February 28?
February has no 31st, so the date moves back to the last day that does exist. In a leap year, the answer is February 29. This keeps the result inside the month you're aiming for, instead of spilling into March.
Does the start date count as day one?
No. The start date is day zero, so adding 1 day gives the following day. If your deadline counts the start date as the first day, enter one day fewer.
Can I skip weekends?
Not here. This tool counts every calendar day, though it does tell you how many are weekdays. The business days calculator adds only Monday-to-Friday days, and it can also allow for holidays.
How do I find a date in the past, such as 60 days ago?
Choose Subtract and enter 60 in the days field. The table also switches to dates before the start. So you can read off 30, 90 or 180 days ago at the same time.
How this calculator is checked
Method: Gregorian calendar arithmetic; month addition with end-of-month clamping as implemented in the shared date engine. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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