iCalendar Data Model

The iCalendar data format has a well defined data model. iCalendar “objects” encompass a set of iCalendar “components” each of which contains a set of iCalendar “properties” and possibly other sub-components. An iCalendar property consists of a name, a set of optional parameters (specified as “key=value” pairs) and a value.

iCalendar components include:

VCALENDAR which encapsulates a set of other components to form a “calendar”
VEVENT which represents an event
VTODO which represents a task or to-do
VJOURNAL which represents a journal entry
VFREEBUSY which represents periods of free or busy time information
VTIMEZONE which represents a timezone definition (timezone offset and daylight saving rules)
VALARM is a sub-component and is used to set alarms or reminders on events or tasks.
STANDARD and DAYLIGHT are sub-components used in VTIMEZONE components to represent standard and daylight time onsets and offsets.

Properties include:

DTSTART which represents a start time for a component
DTEND which represents an end time for a component
SUMMARY which represents a title or summary for a component
RRULE which can specify rules for repeating events or tasks (for example, every day, every week on Tuesdays, etc)
ORGANIZER which represents the calendar user who is organizing an event or assigning a task
ATTENDEE which represents calendar users attending an event or assigned a task

In addition to this data model and the predefined properties, the specification defines how all those are used together to define the semantics of calendar objects and scheduling. The semantics are basically a set of rules stating how all the components and properties are used together to ensure that all iCalendar products can work together to achieve good interoperability. For example, a rule requires that all events must have one and only one DTSTART property. The most important part of the iCalendar specification is the semantics of the calendaring model that it represents. The use of text, XML or JSON to encode those is secondary.

iCalendar was designed to be extensible, allowing for new components, properties and parameters to be defined as needed. A registry exists to maintain the list of standard extensions with references to their definitions to ensure anyone can use them and work well with others.

New components being worked on include VAVAILABILITY and VPOLL.