Country calendars mistakes, and the fixes that hold

Every error here produces a number that looks reasonable. That is what makes them survive: a count that is out by two days in a month reads as a count, not as a fault.

These nine country calendars mistakes each have a symptom that gives them away.

What to take away

Assuming Saturday and Sunday

The plain spreadsheet function has this assumption baked in and does not surface it. A sheet reused for Bangladesh's government-office calendar keeps counting Fridays as open and Sundays as closed, and the answer is wrong in both directions at once. Configure Friday and Saturday as weekend days for that calendar, and verify the convention for the specific obligation or employer.

The symptom is a monthly total that looks right and a short span that does not. Two-rest-day conventions agree over a whole month and disagree over a few days, so a test built on month totals passes. For the calculation method, see the cross-country formula guide.

Assuming there is one national list

A country can have several lists that all describe closures and govern different things. Courts, banks and public offices frequently close on different sets of days, and a payment deadline and a filing deadline are therefore not counted on the same calendar. In the United States, the Office of Personnel Management (OPM) publishes federal holiday schedules, and 5 U.S.C. § 6103 identifies federal holidays; neither automatically supplies every state, court, bank, or employer calendar.

Match the list to the obligation: for U.S. federal-employee holidays, use OPM's federal holiday schedule; for a state or court deadline, use that state's or court's own calendar. A list with no stated scope is not necessarily the right one.

Copying resolved dates for a rule-based closure

A closure defined as a weekday of a month is a rule, not a date. Copying the date it produced last year works most years and fails in the year the rule and the calendar interact differently. For U.S. federal holidays, 5 U.S.C. § 6103(a) gives recurring rules such as Thanksgiving on the fourth Thursday in November; encode the statutory rule rather than copying last year's resolved date.

Treating a country as a timezone

A country can span several zones, and a zone can cross several countries. Deriving a zone from a country code will be wrong for a large minority of the world's population, and the error is a whole number of hours, which makes it look like a data problem rather than a modeling one.

Store an identifier from the tz database with each schedule and let it be independent of the calendar. The two happen to coincide often enough that the mistake survives testing.

Assuming a substitution rule

Some calendars move a closure that falls at a weekend onto a nearby working day; others do not. Assuming substitution overstates the deduction, and assuming its absence understates it, and either way the error appears only in years where a closure lands at a weekend.

For U.S. federal holidays, 5 U.S.C. § 6103(b) provides that a Saturday holiday is observed on the preceding Friday and a Sunday holiday on the following Monday for federal employees' holiday scheduling; OPM's published schedule is a practical check. Do not apply that rule to a state, court, bank, or private calendar unless its own governing authority says so.

Merging several countries' closures into one list

A single range holding every country's dates subtracts every country's closures from every count. The totals come out low in a way nobody can attribute to a country, because the list no longer records which entry belongs to whom.

Keep one range per calendar with a publisher and a retrieval date beside it. For a U.S. state or regional layer, keep its dates identifiable rather than folding them into the federal range; the state and regional calendar guide covers that distinction.

Counting a cross-border deadline as one number

When two parties in different countries share a deadline, two counts exist. Write the governing calendar into the agreement where possible; until it is identified, calculate and label both calendars separately rather than present one unqualified count. Use an intersection or union only if the agreement specifies that rule.

The failure is a single number that does not say which of the three was used, not picking the wrong one.

Using a working-day figure with no calendar attached

A count quoted with no calendar cannot be compared with another count, including its own value from last month. The business day is a legal and commercial category rather than a unit of time, so the phrase alone carries no definition.

Record the calendar identifier and version with every stored count. Without it, a change of calendar looks like a change in the underlying work.

Trusting a year-ahead list to be complete

A list published a year ahead cannot contain a closure announced three weeks before it happens. Systems that treat the annual list as final will be wrong on those dates with nothing to flag it, and the failure is silent because the arithmetic is fine. For U.S. federal holidays, check the current Office of Personnel Management schedule; state, court, bank, and employer calendars require their own current sources.

Recompute against the current list rather than carrying an annual figure forward. The annual base also moves for reasons unrelated to the list; see how annual working-day totals are counted.

Common questions

Which of these is the most common?

The Saturday and Sunday assumption, because it arrives with the tool rather than with the person, and because nothing in the formula shows it.

How do I test a calculator for a second country quickly?

Count one week by hand under that country's convention and compare. Then count a Thursday to Monday span, which is where two-rest-day conventions differ from each other.

Is there a lookup that gives the right calendar from a country code?

Not one that is safe. A country code tells you nothing about which of several national lists governs your particular obligation, and that is the question that matters.

Do these apply to a company operating in one country only?

Most of them, yes. The scope question, the substitution rule and the year-ahead completeness problem all appear inside a single country.