Date Range Generator

Enumerating a date sequence means repeatedly applying a recurrence step to an anchor date until a bound is passed, and the important distinction is between fixed-length steps and calendar steps. Daily, weekly, biweekly, and custom-day intervals are fixed: each is a whole multiple of 86,400 seconds, so a 7-day step always lands on the same weekday and a 14-day step reproduces the 26-period biweekly payroll year that most US employers run. Monthly, quarterly, and yearly steps are calendar operations with no constant length. A month can be 28, 29, 30, or 31 days, a quarter 90 to 92 days, and a year 365 or 366, so a monthly series anchored on the 31st cannot preserve its day-of-month in February and must clamp or overflow, which is why many billing systems refuse to set a renewal day above 28. Weekend filtering is applied after the step, not instead of it. A weekly step with weekends excluded either keeps every occurrence or drops all of them, since the step preserves weekday; a daily step with the same filter produces the familiar five-day working week, roughly 260 to 262 weekdays in a Gregorian year. Neither variant removes public holidays, which are jurisdiction-specific and must be subtracted separately - US federal holidays alone reduce a 261-weekday year to about 250 working days. For output, ISO 8601 extended format (YYYY-MM-DD) is the only one of the four offered here that sorts correctly as plain text, which matters when the list is pasted into a spreadsheet column or a CSV.

Machine-generated date sequences grew out of job scheduling rather than calendars. The cron daemon shipped with Version 7 Unix in 1979 introduced the idea of expressing a repeating schedule as a compact field pattern of minutes, hours, days of month, months, and days of week; Paul Vixie's rewrite, released in 1987, became the implementation most later systems inherited. Its known weakness is instructive: the day-of-month and day-of-week fields interact by union rather than intersection, an ambiguity that shows up whenever a schedule needs both a calendar day and a weekday constraint. Calendar software needed richer rules than cron could express, and the answer was the recurrence rule of the iCalendar specification. Frank Dawson and Derik Stenerson published RFC 2445 in November 1998, defining the RRULE property with FREQ, INTERVAL, BYDAY, BYMONTHDAY, COUNT, and UNTIL parameters, so a series such as the last Friday of every quarter could be stated declaratively; the specification was revised as RFC 5545 in September 2009 and remains the interchange format behind most calendar clients. In parallel, ISO 8601, first issued in 1988 and consolidating earlier ISO recommendations from the 1970s, standardized both the YYYY-MM-DD representation and the repeating-interval notation that writes a series as a start instant, a duration, and a repetition count. Modern date libraries and database generators inherit directly from those two lines of work: a fixed-step engine descended from cron and a calendar-aware rule engine descended from iCalendar.