Date Calculator
Find the exact number of days, weeks, months or years between two dates, or add and subtract days from any date.
Date Calculator — input your values
What does this everyday calculator do?
The date calculator answers two everyday questions: how many days sit between two dates, and what date you get when you add or subtract a number of days from a starting date.
For the difference mode it reports the gap in years, months and days, in weeks, and in total days, and it shows the weekday of the ending date. The add/subtract mode moves a calendar date forward or backward and reports the resulting date, its weekday, and how far it moved from the start.
Formula and variables
Day counts are calendar days: a date span from Jan 1 to Jan 31 is 30 days, because the start date itself is not counted.
| Symbol | Meaning |
|---|---|
| Years | Completed years between the dates |
| Months | Full months past the last year mark |
| Days | Calendar days past the last month mark |
| Total days | Every calendar day, counted one at a time |
Worked example
A project runs from January 1, 2026 to March 15, 2026. How long is it?
- Years: 2026 − 2026 = 0 years.
- Months and days: Jan 1 to Mar 15 = 2 months and 14 days.
- Total days: 31 (January) + 28 (February) + 14 (March) = 73 days.
Answer: The project lasts 73 days, or 10 weeks and 3 days.
Tips and common mistakes
Tips
- For date math on deadlines, count the final date as a working day only if it is not a weekend or holiday.
- Inclusive counting (counting both the start and end date) gives one more day than the difference mode here.
- Use total days rather than months for contracts or billing, since month lengths vary between 28 and 31 days.
- When subtracting many days, the calculator borrows across month and year boundaries automatically.
Common mistakes to avoid
- Forgetting that the span from Jan 1 to Jan 2 is 1 day, not 2, unless you count inclusively.
- Assuming every month has 30 or 31 days — February has 28, or 29 in a leap year.
- Using 365 days for 'a year' when comparing dates across leap years; the calendar count is more accurate.
Date Calculator — frequently asked questions
How many days are between two dates?
The calculator counts every calendar day from the start date up to but not including the end date, so Jan 1 to Jan 11 is 10 days.
What date is 90 days from today?
Choose the add/subtract mode, pick today as the start date and enter 90 — the calculator returns the exact date and weekday.
Does the calculator handle leap years?
Yes. All date arithmetic uses the real calendar, so February gains a day in leap years and the total-day count stays exact.
Why does my count differ from counting on my fingers?
Finger counting often includes the start date. This calculator uses exclusive counting, where the gap between Jan 1 and Jan 2 is one day.
Can I add weeks or months instead of days?
Enter weeks as days (1 week = 7 days). Months vary in length, so for month math it is better to use the difference between two calendar dates.
What if I subtract more days than have elapsed this month?
The calculator borrows from earlier months automatically, so subtracting 45 days from January 10 lands on November 26 of the prior year.