|Internet media type||
|Type of format||Calendar data exchange|
- Not to be confused with the Apple iCal.
iCalendar is a computer file format which allows Internet users to send meeting requests and tasks to other Internet users, via email, or sharing files with an extension of
.ics. Recipients of the iCalendar data file (with supporting software, such as an email client or calendar application) can respond to the sender easily or counter-propose another meeting date/time.
iCalendar is used and supported by a large number of products, including Google Calendar, Apple Calendar (formerly iCal), GoDaddy Online Group Calendar, IBM Lotus Notes, Yahoo! Calendar, Evolution (software), eM Client, Lightning extension for Mozilla Thunderbird and SeaMonkey, and partially by Microsoft Outlook and Novell GroupWise.
iCalendar is designed to be independent of the transport protocol. For example, certain events can be sent by traditional email or whole calendar files can 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.
History and design 
iCalendar was created by 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 by the Internet Mail Consortium (IMC). iCalendar data files are plain text files with the extension
.ifb (for files containing availability information only). RFC 5545 replaced RFC 2445 in September 2009 and now defines the standard.
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.
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). iCalendar data has the MIME content type text/calendar.
Limitations and future 
The iCalendar format is designed to transmit calendar-based data, such as events, and intentionally does not describe what to do with that data. Thus, other programming may be needed to negotiate what to do with this data.
iCalendar is meant to "provide the definition of a common format for openly exchanging calendaring and scheduling information across the Internet". While the features most often used by users are widely supported by iCalendar, some more advanced capabilities have problems. For example, most vendors do not support Journals (VJOURNAL). "[R]ecurring and repeating meetings still have a bit of mystery and ambiguity associated with them. Resulting in no true interoperability between the current calendaring and scheduling vendors." VTODOs have had conversion problems as well.
The memo "Calendar Access Protocol" (RFC 4324) was an initial attempt at a universal system to create real-time calendars. This protocol was eventually abandoned, possibly because of excessive complexity. Regardless, iCalendar-based code such as GroupDAV and CalDAV are now being used more frequently in both client and server software packages.
The IETF "Calendaring and Scheduling Working Group" (ietf-calsify WG) has published additional proposed revisions to the iCalendar standards (as RFC-drafts/"Proposed"; see ), but as of January 2011, the group has 'ended'. Much of their work focused on clarifications to the previous standards, and was apparently influenced on work by the Calendaring and Scheduling Consortium (a.k.a. Calconnect)
Technical specifications 
Core object 
The top-level element in iCalendar is the Calendaring and Scheduling Core Object, a collection of calendar and scheduling information. Typically, this information will consist of a single iCalendar object. However, multiple iCalendar objects can be 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) is made up of a list of calendar properties and one or more calendar components. The calendar properties apply to the entire calendar. The calendar components are several calendar properties which create a calendar schematic (design). For example, the calendar component can specify an event, a to-do list, a journal entry, time zone information, or free/busy time information, or an alarm. Empty lines are not allowed in some versions of usage (Google calendar).
BEGIN:VCALENDAR VERSION:2.0 PRODID:-//hacksw/handcal//NONSGML v1.0//EN BEGIN:VEVENT UID:email@example.com DTSTAMP:19970714T170000Z ORGANIZER;CN=John Doe:MAILTO:firstname.lastname@example.org DTSTART:19970714T170000Z DTEND:19970715T035959Z SUMMARY:Bastille Day Party END:VEVENT END:VCALENDAR
There are many different types of components which can be used in iCalendar, as described below.
Events (VEVENT) 
VEVENT describes an event, which has a scheduled amount of time on a calendar. Normally, when a user accepts the calendar event, this will cause that time to be considered busy. A
VEVENT may include a
VALARM which allows an alarm. Such events have a
DTSTART which sets a starting time, and a
DTEND which sets an ending time. If the calendar event is recurring,
DTSTART sets up the start of the first event.
VEVENT also is used for calendar events without a specific time, such as anniversaries and daily reminders. 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:<Num of Update>
I.e., for the first update:
In Microsoft Outlook, the
SUMMARY corresponds to the "Subject" entry in the "Appointment" form, and
DESCRIPTION to the descriptive text below it. In addition, Outlook 2003 demands a
UID and a
To-do (VTODO) 
VTODO explains 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. 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 SEQUENCE element shows this to-do has been modified twice since it was initially created.
BEGIN:VCALENDAR VERSION:2.0 PRODID:-//ABC Corporation//NONSGML My Product//EN BEGIN:VTODO DTSTAMP:19980130T134500Z SEQUENCE:2 UID:email@example.com ACTION:AUDIO TRIGGER:19980403T120000 ATTACH;FMTTYPE=audio/basic:http://example.com/pub/audio- files/ssbanner.aud REPEAT:4 DURATION:PT1H END:VTODO END:VCALENDAR
Journal entry (VJOURNAL) 
VJOURNAL is a journal entry. They attach descriptive text to a particular calendar date, may 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 programs support VJOURNAL entries, although examples exist: Plum Canary's Chirp software uses VTODO and VJOURNAL together. Also KOrganizer from the KDE desktop supports VJOURNAL.
The following is an example of a journal entry:
BEGIN:VCALENDAR VERSION:2.0 PRODID:-//ABC Corporation//NONSGML My Product//EN BEGIN:VJOURNAL DTSTAMP:19970324T120000Z UID:firstname.lastname@example.org ORGANIZER:MAILTO:email@example.com STATUS:DRAFT CLASS:PUBLIC CATEGORIES:Project Report, XYZ, Weekly Meeting DESCRIPTION:Project xyz Review Meeting Minutes\n Agenda\n1. Review of project version 1.0 requirements.\n2. Definition of project processes.\n3. Review of project schedule.\n Participants: John Smith, Jane Doe, Jim Dandy\n-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. END:VJOURNAL END:VCALENDAR
(Note: This example is literally taken from RFC 2445 with the correction of changing the word 'CATEGORY' to 'CATEGORIES', which is a mistake in the original RFC)
Free/busy time (VFREEBUSY) 
The following is an example of published busy time information.:
BEGIN:VCALENDAR VERSION:2.0 PRODID:-//RDU Software//NONSGML HandCal//EN BEGIN:VFREEBUSY ORGANIZER:MAILTO:firstname.lastname@example.org DTSTART:19980313T141711Z DTEND:19980410T141711Z FREEBUSY:19980314T233000Z/19980315T003000Z FREEBUSY:19980316T153000Z/19980316T163000Z FREEBUSY:19980318T030000Z/19980318T040000Z URL:http://www.example.com/calendar/busytime/jsmith.ifb END:VFREEBUSY END:VCALENDAR
Other component types 
Other component types include VTIMEZONE (time zones) and VALARM (alarms). Some components can include other components (VALARM is often included in other components).
Distributing updates 
The UID field distributes updates when a scheduled event changes. When the event is first generated a globally unique identifier is created. If a later event is distributed with the same UID, it replaces the original one.
Calendar extensions 
vCalendar and iCalendar support private software 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-ALLDAYEVENT - Microsoft Outlook all day event flag
- X-MICROSOFT-CDO-BUSYSTATUS - Microsoft Outlook status information
- X-WR-CALNAME - The display name of the calendar
- X-WR-CALDESC - A description of the calendar
- X-PUBLISHED-TTL - Recommended update interval for subscription to the calendar
vCalendar 1.0 
iCalendar's design was based on the previous file format vCalendar created by the Internet Mail Consortium (IMC).
Here is an example of information in vCalendar format:
BEGIN:VCALENDAR VERSION:1.0 BEGIN:VEVENT CATEGORIES:MEETING STATUS:TENTATIVE DTSTART:19960401T033000Z DTEND:19960401T043000Z SUMMARY:Your Proposal Review DESCRIPTION:Steve and John to review newest proposal material CLASS:PRIVATE END:VEVENT END:VCALENDAR
After iCalendar was released, the Internet Mail Consortium 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."
- iCalendar is a standard (RFC 5545) for calendar data exchange. The standard is sometimes referred to as "iCal", which was the name of the Apple Inc. calendar program before OS X Mountain Lion (see iCal) that provides one of the implementations of the standard.
- From iCal
- From GoDaddy Help Center - Importing, Exporting, and Subscribing to Calendars
- From Lotus Notes 8.5 iCalendar support info on ibm.com.
- 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) explains how iCalendar interacts with other calendar computer language (current and future).
- Klemow, Jason (2005), "Why remove iCalendar Recurring Rules?", From the shores of the Muddy Branch, retrieved July 30, 2012
- CalConnect, 2004
- 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.
- IETF-calsify website: Charter
- "[ietf-calsify]: the end of calsify working group– not the end of the mailing list, http://lists.osafoundation.org/pipermail/ietf-calsify/2010-December/002272.html, retrieved 2010-01-26.
- http://www.calconnect.org/ -- "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 consortiumm."
- From RFC 2445
- Note that Apple iCal and Microsoft Outlook use additional descriptors, as follows, to provide further information about the calendar.
X-WR-CALDESC:Celebrations of various revolutionary activities.
Where the X-WR-RELCALID is a UUID.
- But an event can be set to be "TRANSPARENT" to change this interpretation.
- These events would have a
DATEvalue type for the
DTSTARTproperty instead of the default
DATE-TIME, and need not include a
- From RFC 2445.
- 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.
- From RFC 2445 The iCalendar object might be placed at some URL with the extension ".ifb"
- Some components are often defined to support other components defined after them (VTIMEZONE is often used this way).[clarification needed]
- An example UID might be "Y2007S2C131M5@example.edu", for the 5th meeting of class 131 in semester 2 at a hypothetical college.
- From IMC website.
See also 
- List of applications with iCalendar support
- Scheduling OSID provides a software interface abstraction for using calendaring protocols
- Webcal - an unofficial Uniform Resource Identifier (URI) scheme
- Zip file
- RFC 5545 Internet Calendaring and Scheduling Core Object Specification (iCalendar) (replaces RFC 2445)
- RFC 5546 iCalendar Transport-Independent Interoperability Protocol (iTIP) (replaces RFC 2446)
- RFC 6047 iCalendar Message-Based Interoperability Protocol (iMIP) (replaces RFC 2447)
- RFC 6321 xCal: The XML format for iCalendar (iCalendar XML Representation)
- 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.