ICS (iCalendar) Event Builder

Build a single calendar event as a standard RFC 5545 .ics file you can import into Google Calendar, Apple Calendar, Outlook, Fastmail, or any iCalendar-compatible app. Set the title, start and end times (or mark as all-day), location, organizer, attendees, recurrence rule (RRULE), an optional reminder (VALARM), and a URL. The file is generated on the fly and offered as a download — nothing is uploaded.

An .ics file is a description of an event, not an invitation. Importing one puts the event on your own calendar and sends nothing to anybody, so listing attendees records who is expected rather than asking them. Delivering an actual invitation means emailing the file with the iTIP method set to REQUEST, which is what a calendar server does on your behalf and what a downloaded file cannot do by itself.

All-day events are stored as a date without a time, and their end date is exclusive: a single-day event on the 3rd ends on the 4th. Getting that wrong by a day is the most common bug in hand-written iCalendar, and it is why an event authored elsewhere sometimes shows up spanning one day too few. Timed events here carry an explicit zone, which matters because the same wall-clock time means different instants in different places, and an event written without a zone is interpreted as local to whoever opens it.

Recurrence is the part most likely to surprise. RRULE describes a pattern rather than a list, so the client expands it, and clients disagree at the edges: a monthly rule on the 31st skips the months that have no 31st in some and clamps to the last day in others, and a rule ending with UNTIL needs that timestamp in UTC when the start is zoned.

One file, one event. Importing the same file twice usually updates rather than duplicates, because the UID identifies the event, but an app that ignores the UID will happily create a second copy.