What's VTIMEZONE used for in icalendar? Why not just UTC time?

Question

What's VTIMEZONE used for in icalendar?

e.g. 1

BEGIN:VCALENDAR VERSION:2.0 PRODID:-//Apple Inc.//Mac OS X 10.12.3//EN CALSCALE:GREGORIAN BEGIN:VTIMEZONE TZID:Asia/Shanghai BEGIN:STANDARD TZOFFSETFROM:+0900 RRULE:FREQ=YEARLY;UNTIL=19910914T150000Z;BYMONTH=9;BYDAY=3SU DTSTART:19890917T000000 TZNAME:GMT+8 TZOFFSETTO:+0800 END:STANDARD BEGIN:DAYLIGHT TZOFFSETFROM:+0800 DTSTART:19910414T000000 TZNAME:GMT+8 TZOFFSETTO:+0900 RDATE:19910414T000000 END:DAYLIGHT END:VTIMEZONE BEGIN:VEVENT TRANSP:OPAQUE LAST-MODIFIED:20170305T083916Z UID:[email protected] DTSTAMP:20170305T083916Z LOCATION:Cell Group DESCRIPTION: STATUS:CONFIRMED SEQUENCE:0 SUMMARY:Study Bible DTSTART;TZID=Asia/Shanghai:20170324T193000 DTEND;TZID=Asia/Shanghai:20170324T213000 X-APPLE-TRAVEL-ADVISORY-BEHAVIOR:AUTOMATIC CREATED:20160312T140632Z RRULE:FREQ=WEEKLY;BYDAY=FR BEGIN:VALARM X-WR-ALARMUID:9B47E27E-9063-417E-B488-409387A3201A UID:9B47E27E-9063-417E-B488-409387A3201A TRIGGER;VALUE=DATE-TIME:19760401T005545Z ACKNOWLEDGED:20161125T105826Z X-APPLE-DEFAULT-ALARM:TRUE ACTION:NONE END:VALARM END:VEVENT END:VCALENDAR 

From iCalendar Documentation

VTIMEZONE: provides a grouping of component properties that defines a time zone.

1. What's the definition of VTIMEZONE?

Is a rule to describe winter time and summer time for a location?

2. What's VTIMEZONE used for?

I don't have to specify a VTIMEZONE component in icalendar object. Instead, I transofrm my local time to utc time, and export icalendar event.

# Before DTSTART:20170324T113000Z DTEND:20170324T130000  # After DTSTART;TZID=Asia/Shanghai:20170324T193000 DTEND;TZID=Asia/Shanghai:20170324T213000 

2.1 Does this make sense? a icalendar object without VTIMEZONE

BEGIN:VCALENDAR VERSION:2.0 PRODID:-//Apple Inc.//Mac OS X 10.12.3//EN BEGIN:VEVENT TRANSP:OPAQUE LAST-MODIFIED:20170305T083916Z UID:[email protected] DTSTAMP:20170305T083916Z LOCATION:Cell Group DESCRIPTION: STATUS:CONFIRMED SEQUENCE:0 SUMMARY:Study Bible DTSTART;TZID=Asia/Shanghai:20170324T113000 DTEND;TZID=Asia/Shanghai:20170324T213000 X-APPLE-TRAVEL-ADVISORY-BEHAVIOR:AUTOMATIC CREATED:20160312T140632Z RRULE:FREQ=WEEKLY;BYDAY=FR BEGIN:VALARM X-WR-ALARMUID:9B47E27E-9063-417E-B488-409387A3201A UID:9B47E27E-9063-417E-B488-409387A3201A TRIGGER;VALUE=DATE-TIME:19760401T005545Z ACKNOWLEDGED:20161125T105826Z X-APPLE-DEFAULT-ALARM:TRUE ACTION:NONE END:VALARM END:VEVENT END:VCALENDAR 

2.2 If this makes sense, what's the usage of VTIMEZONE?

3. Consistence between VTIMEZONE and VEVENT

Take e.g. 1 for example.

.... BEGIN:VTIMEZONE TZID:Asia/Shanghai                           <- Part 1 ..... END:VTIMEZONE   BEGIN:VEVENT .... DTSTART;TZID=Asia/Shanghai:20170324T193000   <- Part 2 DTEND;TZID=Asia/Shanghai:20170324T213000     <- Part 3 .... END:VEVENT 

3.1. Must timezone in VEVENT be consistent with VTIMEZONE?

3.2 Can I use other timezone to describe time in part2 and part3?

e.g.

.... BEGIN:VTIMEZONE TZID:Asia/Shanghai                           <- Part 1 ..... END:VTIMEZONE   BEGIN:VEVENT .... DTSTART:20170324T193000Z   <- Part 2 UTC time DTEND:20170324T213000Z     <- Part 3 UTC Time .... END:VEVENT 

Answer 1

1. What's the definition of VTIMEZONE?

VTIMEZONE is used to store the specific rules for the UTC offset a timezone has at any given point in time (i.e. when it is daylight savings time and when it is standard time).

2. What's VTIMEZONE used for?

It's used when you want to format your various timestamps throughout the iCalendar under a specific timezone instead of UTC.

You might ask: Why not just use UTC everywhere? Wouldn't it be simpler? The answer is: timezones are needed because of recurrence rules. If you have a repeating event, and that event crosses a daylight/standard boundary, you have to specify a timezone for that event. If you just used UTC, the event's time would become incorrect once you passed the daylight/standard boundary.

2.1 Does this make sense? a icalendar object without VTIMEZONE

Your example is incorrect.

The DTEND property in your example is referencing a VTIMEZONE component that does not exist ("Asia/Shanghai").

DTEND;TZID=Asia/Shanghai:20170324T213000 

An iCalendar parser will look at that TZID parameter, and then look for a VTIMEZONE component whose TZID property matches it. Because your iCalendar object does not have such a VTIMEZONE, the iCalendar parser technically doesn't know how to interpret that date. The TZID parameter is merely treated as a unique identifier. It does not have any meaning in and of itself.

If you want a date to be formatted under a specific timezone, but you do not want to include a VTIMEZONE component in your iCalendar object, you can use a global timezone ID. Global timezone IDs are different from normal TZID parameters because they begin with a forward slash:

DTEND;TZID=/Asia/Shanghai:20170324T213000 

The downside to global timezone IDs is that the iCalendar specification does NOT specify how these IDs are supposed to be interpreted by the parser. However, in practice, I would imagine that out of all the parsers that support global IDs, most of them probably treat them as Olson IDs.

3.1. Must timezone in VEVENT be consistent with VTIMEZONE?

The timezone(s) defined in the VTIMEZONE component(s) have global scope across the iCalendar object. So any TZID parameter in a VEVENT must reference a VTIMEZONE component (unless it is a global timezone ID).

3.2 Can I use other timezone to describe time in part2 and part3?

Just because your iCalendar object has a VTIMEZONE component, doesn't mean you have to use it. Your example is valid.