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About Dates
Add or subtract years, months and days from a date.
Federal holidays falling on a Saturday are usually observed the preceding Friday, which shifts payroll, banking and courier schedules. Leap seconds and time-zone database updates affect software more than they affect a wall calendar.
US dates run month-day-year, and the span between two dates excludes the start day in most legal and financial counting. Business-day counts skip weekends and the eleven federal holidays.
American dates are written month before day, and a span between two instants is not simply their difference — daylight saving adds or removes an hour twice a year in most states, and leap years add a day every fourth year. Business-day counts skip weekends and the federal holidays that fall inside the range.
Time zones in the United States run from Eastern to Pacific across four mainland bands plus Alaska and Hawaii, so the same instant carries a different clock reading in each.
Calendars accumulate correction: leap years keep the seasons in place and leap seconds keep atomic time aligned with the rotation of the Earth. Date libraries and spreadsheets disagree about whether the first or last day of a span is counted, which shifts contractual deadlines by a day.
Date Calculator sits with the rest of the library's everyday work, and it rests on the same calendar arithmetic the way a manufacturer's label or a timetable does. Its answer is only as good as the assumptions standing behind it.
The quantities it works from — start date, years to add, months to add and days to add — are the ones a real calendar question actually turns on, and each carries a convention of its own, set out below.
Everyday arithmetic in the United States carries conventions that are easy to miss: dates run month before day, distances and weights follow customary units, and prices are quoted before tax. Those conventions make a surprising share of daily sums come out differently from the same question asked elsewhere.
The figures in this section follow American practice unless the subject itself is international.