Component Name

VTIMEZONE

Purpose

Provide a grouping of component properties that defines a time zone.

Format Definition

A "VTIMEZONE" calendar component is defined by the following notation:

 timezonec  = "BEGIN" ":" "VTIMEZONE" CRLF
              *(
              ;
              ; 'tzid' is REQUIRED, but MUST NOT occur more
              ; than once.
              ;
              tzid /
              ;
              ; 'last-mod' and 'tzurl' are OPTIONAL,
              ; but MUST NOT occur more than once.
              ;
              last-mod / tzurl /
              ;
              ; One of 'standardc' or 'daylightc' MUST occur
              ; and each MAY occur more than once.
              ;
              standardc / daylightc /
              ;
              ; The following are OPTIONAL,
              ; and MAY occur more than once.
              ;
              x-prop / iana-prop
              ;
              )
              "END" ":" "VTIMEZONE" CRLF

 standardc  = "BEGIN" ":" "STANDARD" CRLF
              tzprop
              "END" ":" "STANDARD" CRLF

 daylightc  = "BEGIN" ":" "DAYLIGHT" CRLF
              tzprop
              "END" ":" "DAYLIGHT" CRLF

 tzprop     = *(
              ;
              ; The following are REQUIRED,
              ; but MUST NOT occur more than once.
              ;
              dtstart / tzoffsetto / tzoffsetfrom /
              ;
              ; The following is OPTIONAL,
              ; but SHOULD NOT occur more than once.
              ;
              rrule /
              ;
              ; The following are OPTIONAL,
              ; and MAY occur more than once.
              ;
              comment / rdate / tzname / x-prop / iana-prop
              ;
              )

Description

A time zone is unambiguously defined by the set of time measurement rules determined by the governing body for a given geographic area. These rules describe, at a minimum, the base offset from UTC for the time zone, often referred to as the Standard Time offset. Many locations adjust their Standard Time forward or backward by one hour, in order to accommodate seasonal changes in number of daylight hours, often referred to as Daylight Saving Time. Some locations adjust their time by a fraction of an hour. Standard Time is also known as Winter Time. Daylight Saving Time is also known as Advanced Time, Summer Time, or Legal Time in certain countries. The following table shows the changes in time zone rules in effect for New York City starting from 1967. Each line represents a description or rule for a particular observance.

                   Effective Observance Rule

  +-----------+--------------------------+--------+--------------+
  | Date      | (Date-Time)              | Offset | Abbreviation |
  +-----------+--------------------------+--------+--------------+
  | 1967-1973 | last Sun in Apr, 02:00   | -0400  | EDT          |
  |           |                          |        |              |
  | 1967-2006 | last Sun in Oct, 02:00   | -0500  | EST          |
  |           |                          |        |              |
  | 1974-1974 | Jan 6, 02:00             | -0400  | EDT          |
  |           |                          |        |              |
  | 1975-1975 | Feb 23, 02:00            | -0400  | EDT          |
  |           |                          |        |              |
  | 1976-1986 | last Sun in Apr, 02:00   | -0400  | EDT          |
  |           |                          |        |              |
  | 1987-2006 | first Sun in Apr, 02:00  | -0400  | EDT          |
  |           |                          |        |              |
  | 2007-*    | second Sun in Mar, 02:00 | -0400  | EDT          |
  |           |                          |        |              |
  | 2007-*    | first Sun in Nov, 02:00  | -0500  | EST          |
  +-----------+--------------------------+--------+--------------+

Note

The specification of a global time zone registry is not addressed by this document and is left for future study. However, implementers may find the TZ database [TZDB] a useful reference. It is an informal, public-domain collection of time zone information, which is currently being maintained by volunteer Internet participants, and is used in several operating systems. This database contains current and historical time zone information for a wide variety of locations around the globe; it provides a time zone identifier for every unique time zone rule set in actual use since 1970, with historical data going back to the introduction of standard time.

Interoperability between two calendaring and scheduling applications, especially for recurring events, to-dos or journal entries, is dependent on the ability to capture and convey date and time information in an unambiguous format. The specification of current time zone information is integral to this behavior.

If present, the "VTIMEZONE" calendar component defines the set of Standard Time and Daylight Saving Time observances (or rules) for a particular time zone for a given interval of time. The "VTIMEZONE" calendar component cannot be nested within other calendar components. Multiple "VTIMEZONE" calendar components can exist in an iCalendar object. In this situation, each "VTIMEZONE" MUST represent a unique time zone definition. This is necessary for some classes of events, such as airline flights, that start in one time zone and end in another.

The "VTIMEZONE" calendar component MUST include the "TZID" property and at least one definition of a "STANDARD" or "DAYLIGHT" sub-component. The "STANDARD" or "DAYLIGHT" sub-component MUST include the "DTSTART", "TZOFFSETFROM", and "TZOFFSETTO" properties.

An individual "VTIMEZONE" calendar component MUST be specified for each unique "TZID" parameter value specified in the iCalendar object. In addition, a "VTIMEZONE" calendar component, referred to by a recurring calendar component, MUST provide valid time zone information for all recurrence instances.

Each "VTIMEZONE" calendar component consists of a collection of one or more sub-components that describe the rule for a particular observance (either a Standard Time or a Daylight Saving Time observance). The "STANDARD" sub-component consists of a collection of properties that describe Standard Time. The "DAYLIGHT" sub-component consists of a collection of properties that describe Daylight Saving Time. In general, this collection of properties consists of:

* the first onset DATE-TIME for the observance;

* the last onset DATE-TIME for the observance, if a last onset is known;

* the offset to be applied for the observance;

* a rule that describes the day and time when the observance takes effect;

* an optional name for the observance. For a given time zone, there may be multiple unique definitions of the observances over a period of time. Each observance is described using either a "STANDARD" or "DAYLIGHT" sub-component. The collection of these sub-components is used to describe the time zone for a given period of time. The offset to apply at any given time is found by locating the observance that has the last onset date and time before the time in question, and using the offset value from that observance.

The top-level properties in a "VTIMEZONE" calendar component are:

The mandatory "TZID" property is a text value that uniquely identifies the "VTIMEZONE" calendar component within the scope of an iCalendar object.

The optional "LAST-MODIFIED" property is a UTC value that specifies the date and time that this time zone definition was last updated.

The optional "TZURL" property is a url value that points to a published "VTIMEZONE" definition. "TZURL" SHOULD refer to a resource that is accessible by anyone who might need to interpret the object. This SHOULD NOT normally be a "file" URL or other URL that is not widely accessible.

The collection of properties that are used to define the "STANDARD" and "DAYLIGHT" sub-components include:

The mandatory "DTSTART" property gives the effective onset date and local time for the time zone sub-component definition. "DTSTART" in this usage MUST be specified as a date with a local time value.

The mandatory "TZOFFSETFROM" property gives the UTC offset that is in use when the onset of this time zone observance begins. "TZOFFSETFROM" is combined with "DTSTART" to define the effective onset for the time zone sub-component definition. For example, the following represents the time at which the observance of Standard Time took effect in Fall 1967 for New York City:

 DTSTART:19671029T020000

 TZOFFSETFROM:-0400

The mandatory "TZOFFSETTO" property gives the UTC offset for the time zone sub-component (Standard Time or Daylight Saving Time) when this observance is in use.

The optional "TZNAME" property is the customary name for the time zone. This could be used for displaying dates.

The onset DATE-TIME values for the observance defined by the time zone sub-component is defined by the "DTSTART", "RRULE", and "RDATE" properties.

The "RRULE" property defines the recurrence rule for the onset of the observance defined by this time zone sub-component. Some specific requirements for the usage of "RRULE" for this purpose include:

* If observance is known to have an effective end date, the "UNTIL" recurrence rule parameter MUST be used to specify the last valid onset of this observance (i.e., the UNTIL DATE-TIME will be equal to the last instance generated by the recurrence pattern). It MUST be specified in UTC time.

* The "DTSTART" and the "TZOFFSETFROM" properties MUST be used when generating the onset DATE-TIME values (instances) from the "RRULE".

The "RDATE" property can also be used to define the onset of the observance by giving the individual onset date and times. "RDATE" in this usage MUST be specified as a date with local time value, relative to the UTC offset specified in the "TZOFFSETFROM" property.

The optional "COMMENT" property is also allowed for descriptive explanatory text.

Example

The following are examples of the "VTIMEZONE" calendar component:

This is an example showing all the time zone rules for New York City since April 30, 1967 at 03:00:00 EDT.

 BEGIN:VTIMEZONE
 TZID:America/New_York
 LAST-MODIFIED:20050809T050000Z
 BEGIN:DAYLIGHT
 DTSTART:19670430T020000
 RRULE:FREQ=YEARLY;BYMONTH=4;BYDAY=-1SU;UNTIL=19730429T070000Z
 TZOFFSETFROM:-0500
 TZOFFSETTO:-0400
 TZNAME:EDT
 END:DAYLIGHT
 BEGIN:STANDARD
 DTSTART:19671029T020000
 RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU;UNTIL=20061029T060000Z
 TZOFFSETFROM:-0400
 TZOFFSETTO:-0500
 TZNAME:EST
 END:STANDARD
 BEGIN:DAYLIGHT
 DTSTART:19740106T020000
 RDATE:19750223T020000
 TZOFFSETFROM:-0500
 TZOFFSETTO:-0400
 TZNAME:EDT
 END:DAYLIGHT
 BEGIN:DAYLIGHT
 DTSTART:19760425T020000
 RRULE:FREQ=YEARLY;BYMONTH=4;BYDAY=-1SU;UNTIL=19860427T070000Z
 TZOFFSETFROM:-0500
 TZOFFSETTO:-0400
 TZNAME:EDT
 END:DAYLIGHT
 BEGIN:DAYLIGHT
 DTSTART:19870405T020000
 RRULE:FREQ=YEARLY;BYMONTH=4;BYDAY=1SU;UNTIL=20060402T070000Z
 TZOFFSETFROM:-0500
 TZOFFSETTO:-0400
 TZNAME:EDT
 END:DAYLIGHT
 BEGIN:DAYLIGHT
 DTSTART:20070311T020000
 RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=2SU
 TZOFFSETFROM:-0500
 TZOFFSETTO:-0400
 TZNAME:EDT
 END:DAYLIGHT
 BEGIN:STANDARD
 DTSTART:20071104T020000
 RRULE:FREQ=YEARLY;BYMONTH=11;BYDAY=1SU
 TZOFFSETFROM:-0400
 TZOFFSETTO:-0500
 TZNAME:EST
 END:STANDARD
 END:VTIMEZONE

This is an example showing time zone information for New York City using only the "DTSTART" property. Note that this is only suitable for a recurring event that starts on or later than March 11, 2007 at 03:00:00 EDT (i.e., the earliest effective transition date and time) and ends no later than March 9, 2008 at 01:59:59 EST (i.e., latest valid date and time for EST in this scenario). For example, this can be used for a recurring event that occurs every Friday, 8:00 A.M.-9:00 A.M., starting June 1, 2007, ending December 31, 2007,

 BEGIN:VTIMEZONE
 TZID:America/New_York
 LAST-MODIFIED:20050809T050000Z
 BEGIN:STANDARD
 DTSTART:20071104T020000
 TZOFFSETFROM:-0400
 TZOFFSETTO:-0500
 TZNAME:EST
 END:STANDARD
 BEGIN:DAYLIGHT
 DTSTART:20070311T020000
 TZOFFSETFROM:-0500
 TZOFFSETTO:-0400
 TZNAME:EDT
 END:DAYLIGHT
 END:VTIMEZONE

This is a simple example showing the current time zone rules for New York City using a "RRULE" recurrence pattern. Note that there is no effective end date to either of the Standard Time or Daylight Time rules. This information would be valid for a recurring event starting today and continuing indefinitely.

 BEGIN:VTIMEZONE
 TZID:America/New_York
 LAST-MODIFIED:20050809T050000Z
 TZURL:http://zones.example.com/tz/America-New_York.ics
 BEGIN:STANDARD
 DTSTART:20071104T020000
 RRULE:FREQ=YEARLY;BYMONTH=11;BYDAY=1SU
 TZOFFSETFROM:-0400
 TZOFFSETTO:-0500
 TZNAME:EST
 END:STANDARD
 BEGIN:DAYLIGHT
 DTSTART:20070311T020000
 RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=2SU
 TZOFFSETFROM:-0500
 TZOFFSETTO:-0400
 TZNAME:EDT
 END:DAYLIGHT
 END:VTIMEZONE

This is an example showing a set of rules for a fictitious time zone where the Daylight Time rule has an effective end date (i.e., after that date, Daylight Time is no longer observed).

 BEGIN:VTIMEZONE
 TZID:Fictitious
 LAST-MODIFIED:19870101T000000Z
 BEGIN:STANDARD
 DTSTART:19671029T020000
 RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
 TZOFFSETFROM:-0400
 TZOFFSETTO:-0500
 TZNAME:EST
 END:STANDARD
 BEGIN:DAYLIGHT
 DTSTART:19870405T020000
 RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=4;UNTIL=19980404T070000Z
 TZOFFSETFROM:-0500
 TZOFFSETTO:-0400
 TZNAME:EDT
 END:DAYLIGHT
 END:VTIMEZONE

This is an example showing a set of rules for a fictitious time zone where the first Daylight Time rule has an effective end date. There is a second Daylight Time rule that picks up where the other left off.

 BEGIN:VTIMEZONE
 TZID:Fictitious
 LAST-MODIFIED:19870101T000000Z
 BEGIN:STANDARD
 DTSTART:19671029T020000
 RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
 TZOFFSETFROM:-0400
 TZOFFSETTO:-0500
 TZNAME:EST
 END:STANDARD
 BEGIN:DAYLIGHT
 DTSTART:19870405T020000
 RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=4;UNTIL=19980404T070000Z
 TZOFFSETFROM:-0500
 TZOFFSETTO:-0400
 TZNAME:EDT
 END:DAYLIGHT
 BEGIN:DAYLIGHT
 DTSTART:19990424T020000
 RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=4
 TZOFFSETFROM:-0500
 TZOFFSETTO:-0400
 TZNAME:EDT
 END:DAYLIGHT
 END:VTIMEZONE

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