Date & Holiday Methodology

Civil dates and timezones

All calendar arithmetic runs in UTC on civil dates — year, month, and day, with no time component. This means a given square shows the same date whether you open it in Los Angeles or Tokyo. Timezone-dependent arithmetic is the most common source of off-by-one errors in calendar software, and avoiding it entirely is cheaper than handling it correctly.

The month grid

Every monthly view renders exactly 42 cells — six rows of seven. Some months need only five rows, and a 28-day February beginning on the first day of the week needs only four. The grid is fixed at six anyway, so the page never changes height as you move between months, and a stack of printed sheets stays uniform.

Leading and trailing cells are left empty rather than filled with adjacent months’ dates. On paper, a greyed-out 31st from the previous month is a reliable way to write an appointment on the wrong day.

Federal holidays

The eleven U.S. federal holidays follow the schedule published by the Office of Personnel Management. They fall into two kinds:

Fixed-date holidays— New Year’s Day (1 January), Juneteenth (19 June), Independence Day (4 July), Veterans Day (11 November), and Christmas Day (25 December). These land on a different weekday each year.
Floating holidays— Martin Luther King Jr. Day (third Monday in January), Washington’s Birthday (third Monday in February), Memorial Day (last Monday in May), Labor Day (first Monday in September), Columbus Day (second Monday in October), and Thanksgiving (fourth Thursday in November). These always fall on the same weekday.

Actual versus observed dates

A fixed-date holiday that falls on a Saturday is observed on the preceding Friday. One that falls on a Sunday is observed on the following Monday. Floating holidays are always on a weekday, so their actual and observed dates never differ.

Both dates are kept as separate labels throughout the site. An observed date is marked as such, so you can tell the difference between the day a holiday falls and the day offices close for it — which is the distinction that actually matters when you are counting deadlines.

Business days

The default working week is Monday through Friday. A business day is any of those five days that is not an observed federal holiday. The observed date is used rather than the actual date, because that is the day work does not happen.

Add or subtract excludes the start date and steps one calendar day at a time, decrementing the counter only on business days. Adding one business day to a Friday returns the following Monday.
Count between includes both endpoints whenever each is itself a business day. Counting from a Monday to the same Monday returns one.

This is a common planning convention, not a legal interpretation. Court rules, filing deadlines, payroll cycles, and contracts frequently define business days differently — some count calendar days, some exclude the end date, some add state holidays. Always confirm against the rule that governs your situation. Try it on the business day calculator.

Week numbers

Week numbers follow ISO 8601: weeks begin on Monday, and week 1 is the week containing the first Thursday of the year. Under this rule a year has 52 or 53 weeks, and the first days of January can belong to the last week of the previous year. This differs from the North American convention, where week 1 is the week containing 1 January — so a week number here may not match one from a US payroll system.

Scope and limits

U.S. federal holidays only — no state, bank, school, or religious observances.
The Gregorian calendar only.
No daylight-saving transitions, because no times are displayed.
Imported .ics events are read for a start date and a title. Recurrence rules, timezone overrides, and all-day flags are not interpreted.

Sources

U.S. Office of Personnel Management — federal holidays is the authority for the holiday schedule and observance rules. If a date on this site disagrees with it, the site is wrong — please report it.