Days between two dates

What this works out

Two questions hide inside “how far apart are these dates”. The first is how many days there are, which sounds trivial until you notice that leap years, the century rule and two clock changes a year all sit between the answer and a naive subtraction. The second is how many of those days anybody actually works, which is the number a project plan, a notice period or a delivery estimate turns on.

This calculator answers both at once, and shows the working period by period so the total can be checked rather than trusted. The table downloads as a spreadsheet.

The method

Each date is converted into a day number — whole days counted from 1 January 1970 — with integer arithmetic and no clock. That single decision is what makes the result correct. A date on a calendar is not an instant in time, and treating it as one is where date calculators go wrong: 2026-03-29 parsed as UTC midnight and the same date parsed as local midnight are different instants, and in Europe/London the day British Summer Time begins is 23 hours long, so dividing an elapsed-millisecond difference by 86,400,000 reports 30.958 days for a span the calendar shows as 31. With no hour in the arithmetic there is no offset, and with no offset there is nothing for a transition to shift.

The span is a half-open interval. Everything on the page — the day count, the working days, the table, the years-and-months breakdown — is computed from the same [start, end), which is why turning on “count the end date as a day” moves all of them together instead of only the headline.

Working days are counted in closed form rather than by walking the range. Every whole week in a span contributes the same number of worked days, so only the leftover days at the end — at most six — need looking at individually. That matters because the formula runs on every keystroke: a span of three centuries would otherwise be 110,000 iterations per character typed.

Whole months are defined as the largest number of months that can be added to the start date without passing the end, where adding a month clamps to the end of the target month. Stating it as the inverse of the addition is what makes it agree with itself at the awkward edges — the subtract-and-borrow method calls 31 January to 28 February 28 days, while its own month arithmetic puts one month after 31 January squarely on 28 February.

Holidays are subtracted only when they fall on a day that would have been worked. A bank holiday on a Saturday costs nobody a working day, which is the reason substitute Mondays exist in the first place.

Before you read on

1 January to 25 December 2026, end date not counted, with all eight England and Wales bank holidays for 2026 in the holiday box. How many get subtracted?

• AEight — they all fall in 2026.

  They do. Two of them fall outside this span.

• BSix.

  Yes. Christmas Day is the finishing line rather than a counted day, and Boxing Day is past it.

• CSeven — Christmas Day is the last day of the range.

  It is the end of the range, which with the end date not counted means it is not in it.

Six. The span is a half-open interval — everything on the page is computed from the same [start, end) — so 25 December is the finishing line rather than a counted day, and the 28 December substitute is past it. That is also why the switch marked 'count the end date as a day' moves the day count, the working days, the table and the years-and-months breakdown together instead of only the headline figure.

1. 358 = 51 x 7 + 1

   Split the span into whole weeks and a remainder. The remainder is at most six days, whatever the span.

2. 51 x 5 = 255

   Every whole week contributes the same five worked days, whichever days of the week it happens to start on.

3. 255 + 1 = 256

   Only the leftover days need looking at individually. 1 January 2026 is a Thursday, so the one spare day here is a weekday.

4. 256 - 6 = 250

   Subtract only the holidays landing on a day that would have been worked. A bank holiday on a Saturday costs nobody a working day, which is why substitute Mondays exist.

A worked example

From 1 January 2026 to 25 December 2026, end date not counted, with a Saturday and Sunday weekend and the eight England and Wales bank holidays for 2026 in the holiday box.

Days between	358
Business days	250
Weekend days and holidays	108
Weeks	51.14
Whole months	11
In years, months and days	11 months, 24 days
Holidays skipped	6

Check it by hand. 1 January 2026 is a Thursday, and 358 days is 51 whole weeks plus one day:

358 = 51 x 7 + 1

  51 whole weeks x 5 worked days   = 255
  the leftover day is the Thursday =   1
                                     ---
  weekdays                           256
  less bank holidays inside the span -  6
                                     ---
  business days                      250

Six, not eight, because the interval stops before Christmas Day: 25 December is the finishing line rather than a counted day, and 28 December is past it. The weekend total checks the same way — 51 Saturdays and 51 Sundays is 102, plus the six holidays gives 108.

Month by month, which is the table the tool draws and downloads:

Period	Days	Business days	Weekends and holidays
January 2026	31	21	10
February 2026	28	20	8
March 2026	31	22	9
April 2026	30	20	10
May 2026	31	19	12
June 2026	30	22	8
July 2026	31	23	8
August 2026	31	20	11
September 2026	30	22	8
October 2026	31	22	9
November 2026	30	21	9
December 2026	24	18	6

December is short because the range stops on the 25th. The days column sums to 358 and the business column to 250.

These are the same numbers asserted in this tool’s test file, so if the formula ever changes without this page changing with it, the build fails.

What it does not do

It does not know your holidays. The box starts with the England and Wales bank holidays for 2026 because the default has to be something, and every other country, every company shutdown and every regional holiday has to be typed or pasted in. It has no concept of half days, part-time patterns or a working week other than the four weekend options offered, and it does not know that some industries treat a public holiday as an ordinary working day. It reads dates only as YYYY-MM-DD, so 03/04/2026 is refused rather than guessed at — that date is 3 April in London and 4 March in New York, and guessing wrong would be worse than asking. Finally, the calendar is Gregorian all the way back, including before it was adopted, so a span crossing September 1752 in Britain counts the eleven days that were skipped when the country changed over.

And it produces a count, not a page. If what you actually want is something to write the days on, a printable calendar covers the same span in a form that survives a printer.

Every date becomes a day number: whole days counted from 1970-01-01
in the proleptic Gregorian calendar. No hour, no offset, no clock.

  n = daysFromCivil(year, month, day)

The span is one half-open interval, [start, end):

  days = n(end) - n(start),  plus 1 if the end date is counted

Weekday of a day number, 0 = Sunday:

  w = ((n mod 7) + 4 + 7) mod 7        1970-01-01 was a Thursday

Business days over a span of N days beginning on weekday w0, without
walking the range:

  whole   = floor(N / 7)
  left    = N - 7 x whole
  working = whole x (7 - weekendDays)
          + how many of w0, w0+1, ..., w0+left-1 (mod 7) are worked
          - listed holidays inside the span that fall on a worked day

Calendar difference, where addMonths clamps to the end of the target
month (31 January + 1 month = 28 February):

  m     = the largest k with addMonths(start, k) <= end
  days  = n(end) - n(addMonths(start, m))
  years = floor(m / 12),  months = m mod 12