Date Difference Calculator
The number of days between two dates is the base unit everything else (weeks, months, deadlines, project timelines) gets derived from — this counts exactly, rather than approximating from a calendar glance.
Inputs
Result
Select both dates
How the date difference calculator works
Days between two dates = (end date − start date) in milliseconds, converted to whole days.
Weeks = days ÷ 7. Months = days ÷ 30.44, using 30.44 as the average month length across a full year (365.25 ÷ 12), since months themselves vary from 28 to 31 days.
Worked example: January 1 to April 15 (same year, non-leap)
- January (31) + February (28) + March (31) + 15 days into April = 105 days.
- In weeks: 105 ÷ 7 = 15 weeks exactly.
- In months: 105 ÷ 30.44 ≈ 3.45 months.
Common mistakes to avoid
Expecting the 'months' figure to match a calendar month count exactly
Because months vary in length, the months figure here is an average-based approximation (using 30.44 days per month), not a count of how many calendar month boundaries were crossed — for exact calendar-month reasoning, the days or weeks figure is more precise.
Forgetting to check date order, producing an unexpected negative result
If the 'end date' entered is actually earlier than the 'start date', the day count comes out negative — a useful signal that the two dates may have been entered in the wrong fields, rather than an error.
Frequently asked questions
Why is 30.44 used for the average month length instead of just 30?
It's derived from the actual average length of a month across a full year (365.25 days ÷ 12), which more accurately reflects the mix of 28, 29, 30 and 31-day months than a rounder but less accurate flat 30.
Does the day count include or exclude the start and end dates themselves?
It's the pure difference in time between the two dates — from midnight of the start date to midnight of the end date, so 'January 1 to January 2' correctly returns 1 day, not 2.
How is this different from the Working Days Calculator?
This counts every calendar day in the range; the Working Days Calculator specifically excludes weekends, which matters for business or project-timeline contexts where only working days count toward a deadline.
Can this calculate the difference between two dates in different years?
Yes — the calculation works identically regardless of how many years apart the two dates are, correctly accounting for every leap year in between.
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