iCalendar is a standard (RFC 2445) for calendar data exchange. The standard is sometimes referred to as "iCal", which also is the name of the Apple, Inc. calendar program (see iCal) that provides one of the implementations of the standard.

iCalendar allows users to send meeting requests and tasks to other users through email. Recipients of the iCalendar email (with supported software) can respond to the sender easily or counter propose another meeting date/time.

It is implemented/supported by a large number of products. iCalendar data is typically exchanged using traditional email, but the standard is designed to be independent of the transport protocol. For example, it can also be shared and edited by using a WebDav server, or SyncML. Simple web servers (using just the HTTP protocol) are often used to distribute iCalendar data about an event and to publish busy times of an individual. Publishers can embed iCalendar data in web pages using hCalendar, a 1:1 microformat representation of iCalendar in semantic (X)HTML.

Specification Basics

The iCalendar specification is a result of the work of the Internet Engineering Task Force Calendaring and Scheduling Working Group (chaired by Anik Ganguly of Open Text Corporation), and was authored by Frank Dawson of Lotus Development Corporation and Derik Stenerson of Microsoft Corporation. iCalendar is heavily based on the earlier vCalendar industry specification by the Internet Mail Consortium (IMC).

iCalendar data has the MIME content type text/calendar.

The filename extension of "ics" is to be used to designate a file containing (an arbitrary set of) calendaring and scheduling information consistent with this MIME content type.

The filename extension of "ifb" is to be used to designate a file containing free or busy time information consistent with this MIME content type.

The file type code of "iCal" is to be used in Apple Macintosh operating system environments to designate a file containing calendaring and scheduling information consistent with this MIME media type.

The file type code of "iFBf" is to be used in Apple Macintosh operating system environments to designate a file containing free or busy time information consistent with this MIME media type.

iCalendar data files are plain text files. By default, they use the UTF-8 character set; a different character set can be specified using the "charset" MIME parameter (if the transport method used supports MIME, such as Email or []). Each line is terminated by CR+LF (in hexadecimal: 0D0A). Lines should be limited to 75 octets (not characters) long. Where a data item is too long to fit on a single line it can be continued on following lines by starting the continuation lines with a space character (in hex: 20) or a tab character (in hex: 09). Actual line feeds in data items are encoded as a backslash followed by the letter N (the bytes 5C 6E or 5C 4E in UTF-8).

Core object

The top-level object in iCalendar is the Calendaring and Scheduling Core Object. This is a collection of calendaring and scheduling information. Typically, this information will consist of a single iCalendar object. However, multiple iCalendar objects can be sequentially grouped together. The first line must be "BEGIN:VCALENDAR", and the last line must be "END:VCALENDAR"; the contents between these lines is called the "icalbody". The body of the iCalendar object (the icalbody) consists of a sequence of calendar properties and one or more calendar components. The calendar properties are attributes that apply to the calendar as a whole. The calendar components are collections of properties that express a particular calendar semantic. For example, the calendar component can specify an event, a to-do, a journal entry, time zone information, or free/busy time information, or an alarm.

Here is a simple example (from RFC 2445) of an iCalendar object that defines a "Bastille Day Party" event occurring from July 14, 1997 17:00 (UTC) through July 15, 1997 03:59:59 (UTC):

PRODID:-//hacksw/handcal//NONSGML v1.0//EN
SUMMARY:Bastille Day Party

There are many different types of components defined in the standard, as described below. Note that Apple iCal and Microsoft Outlook use additional descriptors, as follows, to provide further information about the calendar.

X-WR-CALNAME:Revolution Parties
X-WR-CALDESC:Celebrations of various revolutionary activities.

Where the X-WR-RELCALID is a UUID.

Events (VEVENT)

A "VEVENT" component provides a grouping of component properties that describe an event that represents a scheduled amount of time on a calendar. Normally, an accepted event will cause that time to be considered busy, but an event can be set to be "TRANSPARENT" to change this interpretation. A VEVENT may include a VALARM calendar component to define alarms. Such events have a DTSTART property that defines its starting time, and a DTEND property defining its ending time. If the event is recurring, DTSTART defines the start of the first event.

Repeated events without a specific time, such as anniversaries and daily reminders, are also represented using VEVENT. These events would have a DATE value type for the "DTSTART" property instead of the default DATE-TIME, and need not include a "DTEND" property.

If you need to send in a cancellation for an event the UID should be same as the original event and the component properties should be set to cancel Ex.


For sending an UPDATE for an event the UID should match the original UID. the other component property to be set is: SEQUENCE:

i.e. for the first update SEQUENCE:1

To-do (VTODO)

The VTODO component describes a to-do item, i.e., an action-item or assignment.

The following is an example of a to-do due on April 15, 1998, from RFC 2445. An audio alarm has been specified to remind the calendar user at noon, the day before the to-do is expected to be completed and repeat hourly, four additional times. The to-do definition has been modified twice since it was initially created.

PRODID:-//ABC Corporation//NONSGML My Product//EN
SUMMARY:Submit Income Taxes

Journal entry (VJOURNAL)

VJOURNAL components describe a journal entry. They simply attach descriptive text notes with a particular calendar date, might be used to record a daily record of activities or accomplishments, or describe progress with a related to-do entry. A "VJOURNAL" calendar component does not take up time on a calendar, so it has no effect on free or busy time (just like TRANSPARENT entries). In practice, few implementations of iCalendar support VJOURNAL entries, though examples exist: Plum Canary's Chirp software uses VTODO and VJOURNAL together as the data format for a project task status management solution. Also KOrganizer from the KDE desktop supports VJOURNAL.

The following is an example of a journal entry, from RFC 2445:

PRODID:-//ABC Corporation//NONSGML My Product//EN
CATEGORY:Project Report, XYZ, Weekly Meeting
DESCRIPTION:Project xyz Review Meeting Minutesn
 Agendan1. Review of project version 1.0 requirements.n2.
 of project processes.n3. Review of project schedule.n
 Participants: John Smith, Jane Doe, Jim Dandyn-It was
  decided that the requirements need to be signed off by
  product marketing.n-Project processes were accepted.n
 -Project schedule needs to account for scheduled holidays
  and employee vacation time. Check with HR for specific
  dates.n-New schedule will be distributed by Friday.n-
 Next weeks meeting is cancelled. No meeting until 3/23.

Free/busy time (VFREEBUSY)

A VFREEBUSY component describes either a request for free/busy time, describes a response to a request, or describes a published set of busy time.

As described in RFC 2445:

When used to request free/busy time information, the "ATTENDEE" property specifies the calendar users whose free/busy time is being requested; the "ORGANIZER" property specifies the calendar user who is requesting the free/busy time; the "DTSTART" and "DTEND" properties specify the window of time for which the free/busy time is being requested; the "UID" and "DTSTAMP" properties are specified to assist in proper sequencing of multiple free/busy time requests.

When used to reply to a request for free/busy time, the "ATTENDEE" property specifies the calendar user responding to the free/busy time request; the "ORGANIZER" property specifies the calendar user that originally requested the free/busy time; the "FREEBUSY" property specifies the free/busy time information (if it exists); and the "UID" and "DTSTAMP" properties are specified to assist in proper sequencing of multiple free/busy time replies.

When used to publish busy time, the "ORGANIZER" property specifies the calendar user associated with the published busy time; the "DTSTART" and "DTEND" properties specify an inclusive time window that surrounds the busy time information; the "FREEBUSY" property specifies the published busy time information; and the "DTSTAMP" property specifies the date/time that iCalendar object was created.

The following is an example of published busy time information from RFC 2445. The iCalendar object might be placed at some URL with the extension ".ifb":

PRODID:-//RDU Software//NONSGML HandCal//EN

Other component types

Other component types defined by the standard include VTIMEZONE (for defining time zones) and VALARM (for defining alarms). Note that some components can include other components (VALARM is often included in other components), and some components are often defined to support other components defined after them (VTIMEZONE is often used this way).

Distributing updates

The UID field is used to facilitate distribution of updates, should a scheduled event change. When the event is first generated a type of globally unique identifier should be generated. If a later event is distributed with the same UID, it is meant to replace the original one. An example UID might be "Y2007S2C131M5@example.com", for the 5th meeting of class 131 in semester 2 at a hypothetical college.

iCalendar Data Exchange

The iCalendar format is designed to describe calendar-based data (like events), and intentionally does not describe what to do with that data. Thus, other protocols may be needed to negotiate what to do with this data.

A companion standard, "iCalendar Transport-Independent Interoperability" (iTIP) (RFC 2446), defines a protocol for exchanging iCalendar objects for the purposes of group calendaring and scheduling between "Calendar Users" (CUs); whoever initiates the exchange of data takes on the role of the "Organizer". This standard defines methods such as PUBLISH, REQUEST, REPLY, ADD, CANCEL, REFRESH, COUNTER (to negotiate a change in the entry), and DECLINE-COUNTER (to decline the counter-proposal).

Another companion standard, "iCalendar Message-based Interoperability Protocol (IMIP)" (RFC 2447), defines a standard method for implementing iTIP on standard Internet email-based transports.

The "Guide to Internet Calendaring" (RFC 3283) describes the relationship of iCalendar to various related standards (current and future).

The iCalendar format was designed to support interoperability of calendar data; while the features most often used by users are widely supported by iCalendar implementations and can be exchanged, interoperation between implementations of some more advanced capabilities is spotty. Most vendors do not support Journals (VJOURNAL). Recurring and repeating meetings still have a "bit of mystery and ambiguity associated with them", causing some interoperability problems in some cases. VTODOs have had interoperability problems as well. (CalConnect, 2004).

iCalendar also is not suitable for calendaring in some non-Gregorian calendar scales such as the lunar calendars used in Israel or Saudi Arabia. Although there exist one-to-one mappings between Gregorian and many other calendar scales, the lack of defined CALSCALE values for those calendars and limitations in various date fields can make native support impossible. For example the Hebrew calendar year may contain either 12 or 13 months, and the Japanese Emperor-based calendar scale contains many eras.

"Calendar Access Protocol" (RFC 4324) was an initial attempt at a universal methodology for implementing real-time calendaring. This protocol was eventually set aside, having never enjoyed any real implementations, possibly due to excessive complexity. Pragmatic iCalendar-based transports such as GroupDAV and CalDAV are now appearing in both client and server software packages, and appear to be gaining traction as common calendar access methods for standards compliant software.

The IETF Calendaring and Scheduling (calsify) Working Group is in the process of revising the iCalendar standards. Nearly all of the work focuses on clarifications to the previous standards, and is often based on work done by another group, the Calendaring and Scheduling Consortium, also known as Calconnect. Formed in 2004, Calconnect addresses interoperability issues through scheduled interoperability tests and through technical committees and surveys to determine actual use cases. Any organization or individual is eligible to join the consortium.

vCalendar 1.0

vCalendar was the precursor of the iCalendar exchange format for calendar data promulgated by the Internet Mail Consortium (IMC).

Here is an example of information in vCalendar format:

SUMMARY:Your Proposal Review
DESCRIPTION:Steve and John to review newest proposal material

After iCalendar was released, IMC stated that it "hopes that all vCalendar developers take advantage of these new open standards and make their software compatible with both vCalendar 1.0 and iCalendar."

Calendar Extensions

vCalendar and iCalendar support private extensions, with a "X-" prefix, a number of which are in common usage.

Some of these include:

  • X-RECURRENCE-ID - vCalendar 1.0 extension which mimics the iCalendar 2.0 RECURRENCE-ID (Nokia S60 3rd Edition)
  • X-EPOCAGENDAENTRYTYPE - defines the client calendar type
  • X-FUNAMBOL-ALLDAY - All Day event flag
  • X-MICROSOFT-CDO-BUSYSTATUS - Microsoft Outlook status information

Programming Libraries

Various program libraries exist that make it easier to create and process iCalendar files:

See also

External links

Internet Mail Consortium (IMC). "Personal Data Interchange: vCard and vCalendar" This describes the relationship of vCard, vCalendar, and iCalendar, and includes IMC's request for implementors to support both vCalendar and iCalendar.

Notes and references

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