ICS Calendar Event Generator

An ICS file is a plain-text calendar payload that follows the iCalendar standard defined by RFC 5545. It begins with a VCALENDAR envelope and contains one or more VEVENT components describing meetings, all-day events, reminders, recurrence rules, and attendees. Almost every modern calendar app understands ICS, which is why it is the de facto interchange format for events between vendors.

A floating time has no timezone at all, so the calendar shows the same wall-clock value no matter where you are. A UTC time is anchored to Coordinated Universal Time and shifts on the display side based on the viewer's offset. A named IANA timezone like Europe/Paris emits a TZID parameter, telling the calendar to interpret the time as a local moment in that specific zone, which is the only approach that survives daylight saving transitions correctly.

RRULE is an iCalendar property that defines a repeating pattern using a FREQ value (DAILY, WEEKLY, MONTHLY, or YEARLY) together with optional modifiers such as INTERVAL, COUNT, UNTIL, BYDAY, BYMONTHDAY, and BYMONTH. The first instance is anchored by DTSTART and every later instance is derived by applying the rule, which keeps the file compact and lets calendar clients expand or collapse the series on demand.

RFC 5545 specifies that when an event uses VALUE=DATE, DTEND points to the first day after the event ends rather than the last day of the event itself. A one-day event on May 1 therefore has DTSTART;VALUE=DATE:20260501 and DTEND;VALUE=DATE:20260502. This convention mirrors how half-open intervals work in computer science and prevents off-by-one errors when calendars compute duration.

UID is a globally unique string identifying a specific event across calendar systems. Two ICS files sharing the same UID are treated as the same event, which is how updates and cancellations propagate: the client receives a new VEVENT with the original UID and a higher SEQUENCE number, then replaces the existing entry. A good UID combines a timestamp with strong randomness and a domain suffix to avoid collisions.

iCalendar requires content lines to be limited to 75 octets, and longer values are folded by inserting a CRLF followed by a single whitespace character. A parser reverses the operation by stripping any whitespace that immediately follows a CRLF. This rule is essential for compatibility with very old mail transports and is one of the most common sources of malformed ICS files in the wild.