Date Difference Calculator
Calculate the days, weeks, months, and years between two dates.
How to use this date difference calculator
- Pick the start date
Select the first date in the Start date field.
- Pick the end date
Select the second date in the End date field.
- Read the results
Review the total days, weeks, approximate months, and approximate years between the two dates.
How this date difference calculator works
This date difference calculator computes the absolute span between two calendar dates and expresses it in several everyday units — total days, weeks, approximate months, and approximate years. The day count is exact because it relies on the Julian Day Number system, which assigns a sequential integer to every calendar day and accounts for all leap-year rules in the proleptic Gregorian calendar. Weeks are derived by dividing total days by seven. Month and year figures are approximate because calendar months vary between 28 and 31 days; the calculator uses the commonly accepted average month length of 30.4375 days (365.25 ÷ 12) and an average year of 365.25 days to give a practical estimate suitable for project planning, contract terms, age verification, and event countdowns.
days = |date₂ − date₁| Suppose you need the span between March 1, 2024 and September 15, 2025. March 1, 2024 is Julian Day Number 2460371 and September 15, 2025 is JDN 2460929. The absolute difference is 2460929 − 2460371 = 564 days. Dividing by 7 gives approximately 80.57 weeks. Dividing by 30.4375 gives approximately 18.53 months. Dividing by 365.25 gives approximately 1.54 years.
If you need the span between January 15, 2025 and October 3, 2025, the calculator returns 261 days, about 37.29 weeks, roughly 8.57 months, and approximately 0.71 years.
- ✓ Both dates follow the Gregorian calendar. The calculator does not adjust for the Julian-to-Gregorian switchover that occurred at different times across countries.
- ✓ Month and year conversions use averaged lengths (30.4375 days per month, 365.25 days per year) and are therefore approximate — they will not exactly match counting whole calendar months or birthdays.
- ✓ The calculation is date-only; it does not factor in time-of-day, so the difference between January 1 at 23:59 and January 2 at 00:01 is reported as one full day.
- ✓ Leap years are handled automatically: years divisible by 4 are leap years, except centuries, which must also be divisible by 400.
- For legal or contractual deadlines that specify whole calendar months, count months on the calendar directly rather than relying on the averaged approximation.
- The result is always a non-negative number regardless of which date you enter first — the calculator takes the absolute value of the difference.
- Business-day counts (excluding weekends and public holidays) require a separate calculation and are not included in this tool.
- If you need age in completed years for legal purposes, use the year and month fields from the two dates rather than the decimal-year approximation.
- Julian Day Number system — U.S. Naval Observatory Astronomical Almanac
- Gregorian calendar leap-year rules — ISO 8601 and the Explanatory Supplement to the Astronomical Almanac
Why the day count is exact
The calculator converts each calendar date to a Julian Day Number — a sequential integer that increases by one for every calendar day regardless of month length or leap-year status. Subtracting the two Julian Day Numbers gives an exact integer day count that never drifts or rounds. This method has been used by astronomers since the sixteenth century precisely because it sidesteps the irregularities of the Gregorian calendar. Whether the span crosses February in a leap year, straddles a century boundary, or covers decades, the subtraction always returns the correct number of elapsed days. Weeks are then derived by dividing that exact count by seven, so the week figure inherits the same precision.
When approximate months and years are good enough
Because calendar months vary between 28 and 31 days, there is no single exact conversion from days to months. The calculator uses the widely accepted average of 30.4375 days per month (365.25 ÷ 12) and 365.25 days per year. These averages are accurate enough for project timelines, lease durations, age estimates, and event planning. They become less useful when you need whole calendar months for billing cycles or legal deadlines — in those cases, count the months on a calendar directly. The decimal-year figure is similarly practical for rough estimates but should not replace an exact birthday or anniversary count when precision matters.
Date difference calculator FAQs
Why are the months and years listed as approximate?
Calendar months range from 28 to 31 days, and years alternate between 365 and 366 days. The calculator uses average lengths (30.4375 days per month and 365.25 days per year) to give a practical estimate, so the result may not align perfectly with counting whole calendar months on a wall calendar.
Does this calculator handle leap years?
Yes. The underlying day count follows full Gregorian leap-year rules — including the century exception — so February 29 is included whenever it falls within the selected date range.
Can I use this to calculate someone's age?
You can get a close approximation by entering the birth date as the start date and today as the end date. For a precise completed-years age, count whole calendar years and months rather than relying on the decimal-year figure.
Does it matter which date I put first?
No. The calculator takes the absolute difference, so you will get the same positive result regardless of whether the earlier or later date is entered first.
Why does the total-days count differ from what I get counting on a calendar?
Fence-post confusion is the most common cause. This calculator counts the number of days between the two dates (exclusive of the start date). If you need to include both endpoints, add one day to the result.