Working days in 2026

How it works

The formula

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

What it assumes

• Dates are treated as days on a calendar, never as instants. There is no time of day anywhere in the arithmetic, so no timezone applies and the answer is the same in Auckland as it is in Anchorage.
• Because there is no clock, a daylight-saving change cannot affect the count. A span across the March transition in Europe/London is 23 hours short of a whole number of 24-hour days, and dividing elapsed milliseconds by 86,400,000 would report one day fewer than the calendar shows.
• By default the end date is the finishing line rather than a counted day, so 1 June to 30 June is 29. Turning on "count the end date as a day" makes it 30, and every figure on the page moves together.
• The order of the two dates does not matter. The tool reports the distance between them and never a negative number.
• Whole months are counted by clamped addition, so 31 January to 28 February is one whole month and 31 January to 1 March is one month and one day. Month arithmetic defined this way is not reversible — a month forward and then a month back from 31 January lands on 28 January.
• A holiday falling on a weekend is not subtracted, because it was not a day that would have been worked. That is exactly why the United Kingdom observes Boxing Day on a Monday when 26 December is a Saturday, and why the substitute date is what the default list carries.
• Every line in the holiday box that has no date on it is ignored rather than refused, because throwing an error would blank the page while somebody is halfway through pasting a list. The holidays-skipped figure is the check on that, since it counts the lines that actually applied.
• The calendar is proleptic Gregorian throughout, including for dates before it was adopted. Britain switched in September 1752 and dropped eleven days doing it, so a span crossing that month is a day count and not what a contemporary diary would have said.

Method written and checked by Tessalor on Jul 30, 2026.