Date Difference Calculator

How to use the date difference calculator

Put a start date in the first box and an end date in the second, and the tool reports the gap between them straight away. Both fields are native date inputs, so you can type the numbers or use your device's calendar picker — whichever is quicker. The example already loaded, January 1 to December 25, 2026, shows the tool working before you touch anything.

The order you enter them in does not matter. The calculator sorts the pair before it counts, so an end date that sits before the start date gives the same answer rather than a negative number or a zero. That means you can paste two dates out of an email in whatever order they appeared and still get the right gap.

The headline number is total days, and it counts the start date but not the end date — the span runs from the start date up to, but not including, the end date. That is how notice periods, rental nights, contract windows, and shipping estimates are normally quoted, and it is why a Monday-to-Tuesday booking is one night, not two. When you want both endpoints included — "how many days am I off, counting my first day and my last day?" — add one to the total yourself.

Underneath the total sit the same span described other ways. The calendar breakdown gives you years, months, and days, which answers a different question from the raw day count: it follows the calendar, so its months are real months of unequal length rather than blocks of thirty days. Weeks and whole months are shown alongside for when a project plan or an invoice is measured in those units.

The last two lines split the span by day of week. Working days counts every Monday through Friday inside the window, and weekend days counts every Saturday and Sunday — together they always add back up to the total. Whatever you change, the address bar updates to match, so copying the URL hands someone else the exact same two dates and the exact same answer.

The formula

The total is an exact day count, not an estimate. Both dates are converted to UTC midnights before subtracting, so a daylight-saving transition in the middle of the span — an hour lost in spring, an hour gained in autumn — cannot round the answer up or down a day the way a local-time subtraction can:

total days = (end at UTC midnight − start at UTC midnight) ÷ 86,400,000 ms
           counts the start date, excludes the end date

whole months = (end.year − start.year) × 12 + (end.month − start.month)
               − 1 if this month's anniversary has not arrived yet
years = whole months ÷ 12 (rounded down) · months = whole months mod 12
days  = day count from that anniversary date to the end date

Worked example with the defaults — Thursday, January 1, 2026 to Friday, December 25, 2026. 2026 is not a leap year, so it runs 365 days: January 1 to December 31 is 364 days, and December 25 is six days earlier, which leaves 358 days. The calendar breakdown anchors on the monthly anniversary — January 1 to December 1 is 11 whole months, and December 1 to December 25 is 24 more days, so 11 months, 24 days. As weeks that is 51 weeks and 1 day, because 51 × 7 = 357 with one day left over.

The working-day split falls out of the same 358: 51 full weeks contribute 51 × 5 = 255 weekdays and 51 × 2 = 102 weekend days, and the single leftover day is Thursday, December 24 — a weekday — so the totals are 256 working days and 102 weekend days, which add back to 358.

The anchored-anniversary method is what keeps the years/months/days breakdown honest at awkward month ends. Counting whole months first and then measuring the remaining days from the clamped anniversary date means January 31 to March 1 comes out as one month and one day, instead of the negative day count you get from borrowing February's 28 days. Leap years need no special case at all: February 29 is simply a day that exists in the subtraction, or does not.

Frequently asked questions