Time & Date
Date Calculator
Use this date calculator to add time to a date or subtract time from it: deadlines, due dates, expiration dates, medication schedules, and project planning.
It applies years and months first using real calendar month lengths (January 31 plus 1 month is February 28, not March 3), then applies weeks and days as exact day counts, and reports the weekday of the result.
The date to shift forward or backward. Defaults to today.
Try an example
Result
Result date
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- ISO date
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- Days shifted (weeks + days portion)
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- Total days from start date
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Student quick launch
Grade planning, algebra checks, and formulas students reach for most.
Study path
Use this calculator with
Follow these when you want the formula behind the answer, a short lesson, or nearby tools in the same topic.
What the date calculator solves
Use this date calculator to add time to a date or subtract time from it: deadlines, due dates, expiration dates, medication schedules, and project planning. It applies years and months first using real calendar month lengths (January 31 plus 1 month is February 28, not March 3), then applies weeks and days as exact day counts, and reports the weekday of the result.
Formula
How to use it
- Pick the start date you want to shift.
- Choose whether to add time or subtract time.
- Enter the years, months, weeks, and days to shift; any of them can stay 0.
- Read the result date and weekday, and check the notes to see whether month-end clamping applied.
How to read the answer
Years and months are applied first as calendar months, keeping your day-of-month whenever possible. When the target month is shorter — January 31 plus 1 month — the day clamps to the last day of that month (February 28, or February 29 in a leap year), and the calculator flags that the clamp fired. Weeks and days are then applied as exact 7-day and 1-day steps using whole-date arithmetic anchored at UTC, so the result never drifts by a day from time zones or daylight saving time. 'Days shifted' shows only the exact-day portion; 'Total days from start date' also includes however many days the year/month step actually moved.
Common mistakes and edge cases
- Adding 1 month is not the same as adding 30 days: calendar months run 28 to 31 days, so 'plus 1 month' keeps the day-of-month while 'plus 30 days' usually does not.
- End-of-month dates clamp: January 31 + 1 month = February 28 (February 29 in a leap year), not March 2 or 3. This matches the common calendar convention used by spreadsheets' EDATE.
- Order matters: this calculator applies years and months before weeks and days. Applying days first can land on a different date when clamping is involved.
- Leap days clamp too: February 29, 2024 plus 1 year is February 28, 2025, because 2025 has no leap day.
- Subtracting can also clamp: March 31 minus 1 month is February 28 or 29, not February 31.
- Enter shift amounts as positive whole numbers and use the Direction control to subtract; negative entries are rejected.
Worked examples
30 days from a date
The typical case: shift a date forward by a pure day count.
Result date
Sunday, June 7, 2026
January 31 plus 1 month (month-end clamp)
February 2026 has 28 days, so day 31 clamps to the 28th.
Result date
Saturday, February 28, 2026
Leap day plus 1 year
February 29, 2024 clamps to February 28, 2025, since 2025 is not a leap year.
Result date
Friday, February 28, 2025
Subtract 15 days across a year boundary
January 10, 2026 minus 15 days lands in late December 2025.
Result date
Friday, December 26, 2025
Mixed shift: 1 month, 2 weeks, and 3 days
Months apply first (June 30 → July 30), then 17 exact days.
Result date
Sunday, August 16, 2026
Zero shift
With every amount at 0, the result is the start date itself.
Result date
Friday, May 8, 2026
Subtract 3 months from May 31 (clamp on the way back)
February 2026 has 28 days, so May 31 minus 3 months clamps to February 28.
Result date
Saturday, February 28, 2026
Invalid calendar date
February 30 does not exist, so the calculator asks for a real date instead of silently rolling into March.
Result date
Error
Frequently asked questions
What is January 31 plus 1 month?+
February 28 (or February 29 in a leap year). When the target month is shorter than your day-of-month, this calculator clamps to the last day of that month instead of spilling into March — the same convention spreadsheets use for EDATE.
What is February 29, 2024 plus 1 year?+
February 28, 2025. 2025 is not a leap year, so the leap day clamps to the 28th. Add 4 years instead and you land back on a real February 29, 2028.
Is adding 4 weeks the same as adding 1 month?+
No. 4 weeks is always exactly 28 days, while a calendar month is 28 to 31 days. Adding 1 month to March 15 gives April 15; adding 4 weeks gives April 12.
Why are years and months applied before weeks and days?+
It matches how people state deadlines ('one month and ten days from signing') and how most calendar tools compute them. Because month-end clamping can fire, applying the day portion first could produce a different final date.
Do time zones or daylight saving time change the result?+
No. The calculator does pure calendar arithmetic on whole dates anchored at UTC, so a 1-day shift is always exactly one calendar day — there is no 23- or 25-hour DST day to cause an off-by-one.
How do I find what date it was 90 days ago?+
Set the start date to today, choose Subtract time, and enter 90 in the Days field. The result shows the date and its weekday.
About this calculator
- Written by
- mathcheck editorial team
- Last reviewed
- September 4, 2026
Method
- Uses the values entered by the user and stable formulas documented on the page.
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Last updated: September 4, 2026