go-icalendar

go-icalendar is an ergonomic wrapper around the iCalendar (RFC 5545) and iMIP/iTIP (RFC 6047 / RFC 5546) formats for Go email clients and schedulers. It turns the low-level property bag exposed by the underlying parser into a flat, render-ready Event struct, and provides the scheduling glue a mail client otherwise has to write by hand.

It was extracted from matcha's mail reader, where it powers the meeting-invite card and the Accept / Decline / Tentative replies sent back to Google Calendar and Outlook.

What it does

• Parse an .ics attachment into one Event (ParseICS) or a whole Calendar of them (Parse), with timezone- and all-day-aware timestamp handling.
• Build outgoing invites — REQUEST, CANCEL and REPLY calendars — and serialize them back to RFC-compliant bytes (Building Invites).
• Reply to a received invite the way Google and Outlook expect, via GenerateRSVP (RSVP Replies).
• Expand recurrences: parse an RRULE and enumerate concrete occurrences in a window (Recurrence).
• Compute availability: merge events into busy intervals and the free gaps between them (Free / Busy).

Features

• Flat Event. Summary, times, organizer, attendees, status, recurrence — all on one struct, no property lookups at the call site.
• Correct timestamps. UTC, floating, TZID-qualified and VALUE=DATE all-day values are handled, including the real-world quirk of a TZID wrongly attached to a date-only value.
• iMIP-correct replies. GenerateRSVP reproduces exactly what schedulers require: METHOD:REPLY, only the responding attendee, updated PARTSTAT, RSVP=TRUE, fresh DTSTAMP, and a preserved UID.
• A real recurrence engine. DAILY/WEEKLY/MONTHLY/YEARLY with INTERVAL, COUNT/UNTIL, BYMONTH, BYMONTHDAY, BYDAY (ordinals like 2MO / -1FR) and BYSETPOS, plus RDATE/EXDATE merging.
• Single dependency. Only github.com/arran4/golang-ical.

Sister projects