iCal (iCalendar)
iCal is short for iCalendar, the open standard format (RFC 5545) that calendar apps use to exchange events. An iCal file is a plain-text .ics file, and an iCal link is a URL that points to one, so Google Calendar, Outlook, Apple Calendar and nearly every other calendar can import the events once or subscribe to them and stay updated. iCal was also the name of Apple's Mac calendar app until Apple renamed it Calendar in 2012, which is why the word has both meanings.
Understanding iCal (iCalendar)
The iCalendar standard was published in 1998 and has become the universal language of calendar data. An .ics file (or iCal feed) contains structured text describing calendar events, including their title, start and end times, recurrence rules, attendees, location, and description. Any calendar application that supports iCalendar can read, import, and subscribe to these files regardless of which platform created them.
The standard supports two primary use cases. First, static .ics file export/import: you download a file from one calendar and import it into another. Second, live calendar subscriptions: your calendar application subscribes to a URL that returns an up-to-date .ics file, which it refreshes periodically so changes made at the source propagate to subscribers.
For personal productivity, iCal is the mechanism that makes cross-platform calendar sharing possible. Adding a conference schedule, sports team fixtures, or public holiday calendar to your personal calendar almost always uses an iCal subscription URL. For developers, the iCalendar format is essential for building anything that creates, reads, or syncs calendar events across platforms.
Common iCalendar fields include DTSTART and DTEND for event times, RRULE for recurrence patterns (e.g., 'every Monday at 9am'), ATTENDEE for participant email addresses, and ORGANIZER for the event creator. VTIMEZONE components handle time zone conversions, which are a frequent source of bugs in calendar integrations.
What is inside an .ics file: it is plain text you can open in any editor. It starts with BEGIN:VCALENDAR, then VERSION:2.0 and a PRODID line naming the app that wrote it. Each event sits in its own block from BEGIN:VEVENT to END:VEVENT, with a UID (a stable ID, so an update replaces the right event), DTSTAMP, DTSTART, DTEND and SUMMARY, plus optional LOCATION, DESCRIPTION, RRULE and ATTENDEE lines. END:VCALENDAR closes the file. Every property is one NAME:value line, and lines longer than 75 octets fold onto a next line that starts with a space. A time ending in Z is UTC; any other time carries a TZID parameter that points to a VTIMEZONE block.
Using iCal in Google Calendar, Outlook and Apple Calendar: in Google Calendar you add a feed under Other calendars with the From URL option, and you load a file through Import & export in settings. Outlook calls a subscribed feed an Internet calendar, and the meeting invitations it sends and receives are .ics attachments, which is what Outlook means by an iCalendar file. On a Mac, Calendar subscribes through File, New Calendar Subscription. A webcal:// link is the same feed served over HTTP; the webcal scheme only tells your device to open it in the calendar app.
iCal connection vs API connection: booking tools, project trackers and school portals often offer an iCal connection, which is a private feed URL your calendar reads. It is one-way and read-only. Your calendar app decides how often to fetch it again, so changes can take hours to show up, you cannot edit or RSVP to those events, and anyone who has the URL can read the feed, so treat it like a password. An API (direct) connection signs in to the calendar provider with OAuth, for example the Google Calendar API or Microsoft Graph for Outlook. It works both ways: the app can create, move and delete events, and changes appear within minutes. Use an iCal feed to publish or follow a schedule; use an API connection when a tool needs to manage your calendar.
iCal is not CalDAV: iCalendar is the data format, while CalDAV (RFC 4791) is the protocol that servers such as iCloud and Fastmail use to sync calendars in both directions, carrying iCalendar data underneath. Apple's old iCal Server product was a CalDAV server, not a different file format.
How GAIA Uses iCal (iCalendar)
GAIA uses direct API connections, not iCal feeds. Connect Google Calendar or Outlook and GAIA can read, create, move and delete events in those calendars. A feed you have subscribed to inside Google Calendar (public holidays, a sports schedule, a shared team calendar) is one of your calendars there, so GAIA sees those events too and accounts for them when it plans your day or flags a conflict. GAIA does not import .ics files or subscribe to iCal URLs by itself: add the feed to your calendar app first.
Related Concepts
AI Calendar Management
AI calendar management is the use of artificial intelligence to schedule, organize, and optimize your calendar by finding ideal meeting times, protecting focus blocks, preparing meeting briefs, and coordinating events with your tasks and communications.
Calendar Automation
Calendar automation uses AI to intelligently manage your schedule by finding optimal meeting times, preparing briefings, blocking focus time, and coordinating calendar events with your tasks and communications.
Meeting Scheduling
Meeting scheduling is the process of choosing a time, duration, and location for a meeting that works for every required attendee, including sending the invite and handling changes.
Time Blocking
Time blocking is a scheduling method where you divide your day into dedicated blocks of time, each assigned to a specific task or type of work, turning your calendar into a concrete plan for the day.
Smart Scheduling
Smart scheduling is the automatic selection of meeting times based on attendee availability, time zones, priorities, and work patterns, instead of manual back-and-forth over email.
