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[iCalendar] - What is the exact meaning of TZOFFSETFROM and TZOFFSETTO and how to calculate/use it?


pixi
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While this may seem slightly of topic from pure FileMaker it may come in handy for creating and exporting .ics-files.

To build an .ics-file which works across timezones they have to contains timezone components. Two of them are TZOFFSETFROM and TZOFFSETTO. While there is an official description here: https://tools.ietf.org/html/rfc5545#section-3.6.5 [Page 65] I can't wrap my head around the meaning and how to calculate it. 

I'm looking for a profound explanation what exactly the difference between both of them is and how to calculate/use it.

I have a list of all timezones and their related UTC_offset_StandardTime and UTC_offset_DaylightSavingTime but there is no "…onset…"-time.
What am I missing?

 

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1 hour ago, pixi said:

I'm looking for a profound explanation what exactly the difference between both of them is and how to calculate/use it.

I believe you can find the explanations later in the same document:

3.8.3.3. Time Zone Offset From:

Quote

Purpose:  This property specifies the offset that is in use prior to
      this time zone observance.

3.8.3.4. Time Zone Offset To:

Quote

 Purpose:  This property specifies the offset that is in use in this
      time zone observance.

 

1 hour ago, pixi said:

I have a list of all timezones and their related UTC_offset_StandardTime and UTC_offset_DaylightSavingTime

I don't think you have sufficient data to calculate a full timezone specification. You need  something like the Olson database that lists the full history of timezone offsets for each region.

 

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Yes, meanwhile I understood why I couldn't get it. I thought the FROM and TO describe a DURATION while they just describe a switch in time.

51 minutes ago, comment said:

I don't think you have sufficient data to calculate a full timezone specification. You need  something like the Olson database that lists the full history of timezone offsets for each region.

 

Well, I have https://en.wikipedia.org/wiki/List_of_tz_database_time_zones which gives me the "TZ database name" (which I guess is the official one, understandable by all system reading an .ics) and the related offset. Those value I need.

 

Or am I missing something?

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What you have gives you only the current offsets. It provides no clue to what the offset was when the local date was February 28, 1970, the local time was 15:30:45 and the nearest city was New York in America. That is what the Olson database is designed to answer - and the VTIMEZONE property is supposed to provide the same information.

I suppose you could construct a minimal VTIMEZONE that covers only the dates and times included in the file, so that the missing information would not be missed. But that is just guessing on my part.

 

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