Date Difference
Days, weeks, months, and ISO week number between dates
This calculator answers one question exactly and three questions approximately, and it is worth knowing which is which before you paste a figure into a contract. The exact answer is how many whole days separate two calendar dates. The approximate answers are how many weeks, months and years that is.
Weeks, months and years come back as decimals — 89.4 weeks, 20.6 months, 1.71 years — rather than as a tidy "1 year, 8 months and 16 days". That is not a shortcut. "Months between two dates" genuinely has no single correct value, and a decimal is an honest way of saying so.
Order does not matter: the result is the absolute distance, so putting the later date first gives the same numbers rather than a negative. Both dates are read at local midnight, which keeps the day count from shifting under anyone in a timezone west of Greenwich.
Four numbers, only the first of them exact
- Days Apart — whole calendar days between the two dates. This is the number to quote. It is exclusive: 1 January to 2 January is 1, not 2.
- Weeks — days divided by 7, to one decimal place. The only one of the three approximations with a fixed, unarguable divisor.
- Months (approx) — days divided by 30.4375, to one decimal place.
- Years (approx) — days divided by 365.25, to two decimal places.
The word "approx" on the last two rows is load-bearing. 20.6 months does not mean "twenty months and eighteen days on a calendar"; it means the gap is 20.6 average-length months long. For anything where the answer feeds a payment schedule or a notice period, use Days Apart and count the calendar months yourself.
The results panel also labels each of the two input dates with its ISO 8601 week number (Week 01 through Week 52 or 53) — useful for aligning a date range to a fiscal or sprint calendar that counts in weeks rather than months. ISO weeks always start on Monday, and Week 01 is defined as the week containing the year's first Thursday, which occasionally puts the first days of January into the last week of the previous year. For the full mechanics and edge cases, see ISO Week Numbering Explained.
Worked example: 12 March 2025 to 28 November 2026
Enter the two dates and the panel reads:
- Days Apart: 626 — exact, and the figure to use for anything contractual.
- Weeks: 89.4 — 626 ÷ 7 = 89.43.
- Months (approx): 20.6 — 626 ÷ 30.4375 = 20.57.
- Years (approx): 1.71 — 626 ÷ 365.25 = 1.714.
Walk the same span on a wall calendar and you get "1 year, 8 months and 16 days". That is a different sentence from 20.6 months, and both are defensible: 8 months of calendar plus 16 days is 8.5-ish months by one reading and 8.53 by another, depending on which months you crossed. February 2026 has 28 days and March has 31, so the two months you happened to cross change the answer. The 626 does not change.
Why 30.4375 and 365.25, rather than 30 and 365
365.25 is the Julian year: three ordinary years of 365 days plus one of 366, averaged. Divide that by twelve and you get 30.4375, the average month length used for the months row. Using the round numbers instead would be visibly wrong. A gap of exactly 365 days is 1.00 years, but divided by 30 it comes out as 12.17 "months" — a 1.4% overstatement that compounds every time somebody multiplies it back up.
365.25 is itself a small simplification. The Gregorian mean year is 365.2425 days, because century years are only leap years when they divide by 400 — 1900 was not, 2000 was. The 0.0075-day-per-year difference is invisible at two decimal places over an ordinary span; over four centuries it accumulates to three days, which is precisely the correction the Gregorian rule exists to make.
The days figure itself is computed by taking both local-midnight timestamps, subtracting, dividing by 86,400,000 milliseconds, and rounding to the nearest whole number. Rounding rather than truncating is what keeps the answer right across a daylight-saving boundary, where the interval between two local midnights is 23 or 25 hours instead of 24.
Fenceposts, clocks, and the month problem
Add one when both ends are consumed. Leave booked from Monday to Friday is five days of absence, but the dates are four days apart. Same for a hotel stay: four nights, five dates. The calculator reports elapsed days; whether the final day counts is a question about your contract, not about the calendar.
Daylight saving does not corrupt the count. In the United States, clocks go forward on the second Sunday of March — 8 March in 2026 — so the interval from local midnight on 8 March to local midnight on 9 March is 23 hours, not 24. A calculator that truncated instead of rounding would report that as 0 days. This one reports 1. The same logic keeps a 100-day span from silently becoming 99.
The month problem, restated. Ask any two systems what falls one month after 31 January and you get different answers: a spreadsheet's EDATE clamps to 28 February, JavaScript's setMonth overflows to 3 March. Because there is no neutral answer, this tool refuses to invent one and gives you an average-month decimal instead.
Weekends and holidays are included. Every calendar day counts here, including the ones nobody works. For a business-day figure use the Working Days Calculator, which also takes a holiday list.
What the divisors mean in days
| Unit as this tool defines it | Days | Where it will disagree with a calendar |
|---|---|---|
| 1 week | 7 | Never — a week is exactly 7 days. |
| 1 month | 30.4375 | By up to 2.4 days on February, 0.6 days on a 31-day month. |
| 1 quarter (3 months) | 91.3125 | Q1 of a common year is 90 days; Q3 is 92. |
| 1 year | 365.25 | By 0.25 days in a common year, 0.75 the other way in a leap year. |
| 1 decade | 3,652.5 | By 0.5 or 1.5 days depending on how many leap years the decade caught. |
Read that middle column as the size of the error you are accepting when you quote the decimal rather than the day count. For a rough sense of scale it is nothing. For a per-diem rate, a notice period or an interest accrual, it is the difference between right and nearly right.
Where the exact day count earns its keep
Interest and per-diem calculations run on day-count conventions with names like ACT/365, ACT/360 and 30/360, and in the ACT conventions the numerator is exactly the figure in the Days Apart box — actual elapsed days, leap years included. The 30/360 conventions deliberately pretend every month has 30 days, which is why a bond coupon and a bank statement can disagree by a day or two on the same period.
Rolling-window immigration rules are the other place the exact count matters and the fencepost bites: a 90-days-in-any-180 allowance counts days of presence, so both your arrival and departure dates are consumed and you add one to the figure here. Statutory notice periods work the same way — "not less than 30 days' notice" is 30 clear days, which usually means serving it 31 dates ahead.
For a gap that starts at a birth date, the Age Calculator gives you completed years and months instead. For the working-day version of a project timeline, see Working Days and the background in How Many Working Days Are Really in a Year?.
Frequently asked questions
Does the count include both the start and end dates?
No. It reports elapsed days, so 1 January to 2 January is 1. If both dates are consumed — leave, hotel nights billed by date, a trip counted for a visa allowance — add one.
Why are months and years shown as decimals instead of a calendar breakdown?
Because a calendar breakdown requires picking a convention for partial months, and the two common conventions disagree by up to three days around month ends. Rather than hide that choice, the tool reports average-length months and leaves the exact day count unambiguous.
Where does 30.4375 come from?
It is 365.25 divided by 12 — the average length of a month in a calendar that averages 365.25 days per year.
Does the order of the two dates matter?
No. The result is the absolute distance, so entering the later date first gives the same four numbers rather than negatives.
What does the ISO week number next to each date mean?
It's the ISO 8601 week (Week 01 through 52 or 53) that date falls in. ISO weeks run Monday to Sunday, and Week 01 is whichever week contains the year's first Thursday — which can place the first few days of January in the final week of the previous year.
Is the count affected by daylight saving time?
Not in the answer. The underlying interval really is 23 or 25 hours across a clock change, but the result is rounded to the nearest whole day, so a one-day gap across the change still reads as 1.
How do I exclude weekends?
Use the Working Days Calculator instead. It walks the range day by day, skips Saturdays and Sundays, and also lets you paste a list of holiday dates to subtract.