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Part of Days between dates: six honest answers to one simple-looking pair
Days between dates holidays: past and future windows
How a closure changes a days between dates count, and why a window that has closed and one that has not must be counted against different lists.
A closure inside the window takes one working day out of the count, and nothing else about the count changes. The hard part is knowing which closures were inside the window, on whose calendar, and as of when.
Whether the window has already happened decides the rest. A closed window is counted against the list that was in force then. An open one is counted against a forecast.
What to take away
- A window in the past is counted against the list as it stood then. Recounting it with today's list is a different calculation.
- A window in the future is counted against a partly forecast list, so the count should carry an as-of date and be repeated nearer the time.
- One working-day closure inside the span subtracts exactly one. A closure on a rest day subtracts nothing unless the calendar substitutes it onto a working day.
- Two parties with two calendars have two counts for the same window. The document decides which one governs; the arithmetic cannot.
Counting a window that has closed
A window that has already closed has one right answer: the count against the list that was in force during the window. If a closure was declared during the window, it was a closure. If a list has since been amended, the amendment does not reach back.
Recomputing an old count with today's list is a different calculation and can contradict the one everybody relied on at the time. Store the list's identity and its as-of date beside the count, as a versioned closure list does, so a later challenge can be answered by looking rather than rebuilding.
Counting a window that has not opened
A window that has not opened is counted against a list that is partly a forecast. Fixed and formula-driven closures are reliable; declared ones exist only from the moment of declaration.
A count made months ahead should carry the as-of date of its list and a note to recount when the annual instrument for the year is issued and again before the window opens.
For the United States federal set, the instrument is 5 U.S.C. 6103, and enacted text is published through official channels like the Government Publishing Office's Statutes at Large collection. Read the instrument to learn whether a weekend closure is substituted onto a working day.
Past vs future windows
Past window
- List used
- In force then
- Amendments
- Do not reach back
- Reliability
- One right answer
- Required note
- As-of date
Future window
- List used
- Partly forecast
- Amendments
- May change count
- Reliability
- Recount needed
- Required note
- As-of date + recount
What one closure does
Monday 1 March to Monday 15 March 2027, both ends counted, gives 11 working days on a Saturday-Sunday calendar. Close Wednesday, 10 March, and it drops to 10. Close Saturday 6 March instead, and it stays 11, since that Saturday was never counted.
One closure, four outcomes
Closure
- None
- 11
- Wednesday 10th
- 10
- Saturday 6th
- 11
- Monday 15th (end)
- 10
- Wed 10th + Fri 12th
- 9
Working days
- None
- Wednesday 10th
- Saturday 6th
- Monday 15th (end)
- Wed 10th + Fri 12th
Close an endpoint, the 15th itself, and the count falls to 10 by losing that endpoint.
In the United States, 4 July 2027 falls on a Sunday, so the observed federal holiday is Monday 5 July 2027. A window that includes 5 July 2027 loses one working day on the federal calendar.
Two calendars, one window
A count has no owner until you say whose calendar it runs on. For a deadline, the receiving body's. For a payment, the payment system's. For a staffing plan, the employer's. For a contract term, whatever the contract names, and if it names nothing, two honest counts.
Level matters. A national list will not carry a regional closure, and a public-office list is not a court's or a bank's.
A supplier and a customer in different regions counting the same fortnight will get different numbers whenever a closure falls on one calendar and not the other. Neither is wrong. The document that created the obligation either names a calendar, in which case that count governs, or it does not, in which case the difference is the dispute.
The fix is drafting, not arithmetic. Name the publisher and region. Put the resulting count or date next to the interval.
Record the list with every count
Five items turn a count into something that survives a challenge.
- The body whose calendar governs the window. If two bodies are involved, name both and count twice.
- That body's own published list, or the official list it says it follows.
- A check that the list covers the whole window. A list that ends before the window treats the uncovered part as fully open.
- The as-of date written on the list, with a copy stored beside the count.
- For a future window, a date to count again.
In a spreadsheet, NETWORKDAYS(start, end, Closures) performs the subtraction once the list is in the Closures range. The function counts both the start and end dates if they are working days. The functions and their endpoint rules are worked separately.
The six readings a pair of dates can carry before any closure is applied are under counting between two dates.
Common questions
A closure was declared mid-window. Does the count change?
Yes, if the calendar is the list as published from time to time. No, if the document fixed the list as of a date before the declaration. Both are workable; silence in the document is not.
Do I subtract a closure that falls on the start date?
If the start date was in the count, yes: a closed start date is not a working day. If your convention already excluded the start, there is nothing to subtract.
Two lists disagree about whether a day was closed. Which wins?
The publisher's own document beats any copy. If both are publishers, the document that created the obligation decides which publisher governs.
Why does a closure change the count here but move the date in a forward calculation?
Because here both dates are fixed and the count is the output. In a forward calculation the count is fixed and the date is the output. A closure always removes one working day; what that does to the answer depends on which quantity was free to move.
