ICaL

iCal, short for iCalendar, is a widely used data format for representing calendar information that can be exchanged between applications and services. The primary file extension is .ics, and the format is plain text, organized into VCALENDAR components that describe events, tasks, journal entries, and availability.

The iCalendar standard was developed by the Internet Engineering Task Force (IETF). It was originally published as RFC 2445 and later superseded by RFC 5545, which defines the core data model and properties. RFC 5546 describes the transport mechanisms for scheduling messages, including iTIP and iMIP.

An iCalendar file uses a text-based syntax with BEGIN and END blocks. The calendar body commonly contains components such as VEVENT (events), VTODO (tasks), VJOURNAL (notes), VFREEBUSY (busy-time information), and VTIMEZONE (time zone descriptions). Key properties include UID, DTSTAMP, DTSTART, DTEND or DURATION, SUMMARY, DESCRIPTION, LOCATION, and STATUS. Recurring events are described with RRULE, with additional information in RDATE and EXDATE. Time zones are represented by VTIMEZONE blocks or references to UTC.

Usage and interoperability: many calendar applications and services—such as Google Calendar, Apple Calendar, Microsoft Outlook, and Thunderbird with Lightning—support importing and exporting ICS data and also use ICS for calendar sharing and scheduling via iTIP or iMIP when requesting or sending updates.

Common considerations: ensure correct time zone handling, folding long lines (per RFCs), and note that some advanced recurrence or attendee features may vary in support across clients. Libraries exist in many languages (for example ical.js, ical4j, and ical.net) to parse and generate ICS data.