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<p>This raises a whole bunch of issues...</p>
<p>First - there is no difference in evaluation of rules - whatever
the context.<br>
</p>
<p>Some clients and servers send a compressed timezone definition
which is only valid for the date/time range of the associated
event.</p>
<p>Most clients and servers ignore the timezone definition anyway.
We carried out a number of tests some time back and as long as
it's a recognized id it's usually ignored.</p>
<p>We did this as a run-up to this spec -
<a class="moz-txt-link-freetext" href="https://tools.ietf.org/html/rfc7809">https://tools.ietf.org/html/rfc7809</a> which allows caldav servers to
optionally skip the tz definition.</p>
<p>Not only is it usually ignored but it's often bigger than the
actual event.</p>
<p>So I guess one question is - does that tz def actually work for
the event date in question? And does that match what you'd get if
you used an up to date full spec?<br>
</p>
<p>This issue - among other things was behind tzdist -
<a class="moz-txt-link-freetext" href="https://tools.ietf.org/html/rfc7808">https://tools.ietf.org/html/rfc7808</a><br>
</p>
<p>Just for ref below is an (oldish) version of the spec for that
zone</p>
<pre>BEGIN:VCALENDAR
PRODID:-//tzurl.org//NONSGML Olson 2018g-rearguard//EN
VERSION:2.0
BEGIN:VTIMEZONE
TZID:Europe/London
TZURL:<a class="moz-txt-link-freetext" href="http://tzurl.org/zoneinfo/Europe/London">http://tzurl.org/zoneinfo/Europe/London</a>
X-LIC-LOCATION:Europe/London
BEGIN:DAYLIGHT
TZOFFSETFROM:+0000
TZOFFSETTO:+0100
TZNAME:BST
DTSTART:19810329T010000
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:+0100
TZOFFSETTO:+0000
TZNAME:GMT
DTSTART:19961027T020000
RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU
END:STANDARD
BEGIN:STANDARD
TZOFFSETFROM:-000115
TZOFFSETTO:+0000
TZNAME:GMT
DTSTART:18471201T000000
RDATE:18471201T000000
END:STANDARD
BEGIN:DAYLIGHT
TZOFFSETFROM:+0000
TZOFFSETTO:+0100
TZNAME:BST
DTSTART:19160521T020000
RDATE:19160521T020000
RDATE:19170408T020000
RDATE:19180324T020000
RDATE:19190330T020000
RDATE:19200328T020000
RDATE:19210403T020000
RDATE:19220326T020000
RDATE:19230422T020000
RDATE:19240413T020000
RDATE:19250419T020000
RDATE:19260418T020000
RDATE:19270410T020000
RDATE:19280422T020000
RDATE:19290421T020000
RDATE:19300413T020000
RDATE:19310419T020000
RDATE:19320417T020000
RDATE:19330409T020000
RDATE:19340422T020000
RDATE:19350414T020000
RDATE:19360419T020000
RDATE:19370418T020000
RDATE:19380410T020000
RDATE:19390416T020000
RDATE:19400225T020000
RDATE:19460414T020000
RDATE:19470316T020000
RDATE:19480314T020000
RDATE:19490403T020000
RDATE:19500416T020000
RDATE:19510415T020000
RDATE:19520420T020000
RDATE:19530419T020000
RDATE:19540411T020000
RDATE:19550417T020000
RDATE:19560422T020000
RDATE:19570414T020000
RDATE:19580420T020000
RDATE:19590419T020000
RDATE:19600410T020000
RDATE:19610326T020000
RDATE:19620325T020000
RDATE:19630331T020000
RDATE:19640322T020000
RDATE:19650321T020000
RDATE:19660320T020000
RDATE:19670319T020000
RDATE:19680218T020000
RDATE:19720319T020000
RDATE:19730318T020000
RDATE:19740317T020000
RDATE:19750316T020000
RDATE:19760321T020000
RDATE:19770320T020000
RDATE:19780319T020000
RDATE:19790318T020000
RDATE:19800316T020000
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:+0100
TZOFFSETTO:+0000
TZNAME:GMT
DTSTART:19161001T030000
RDATE:19161001T030000
RDATE:19170917T030000
RDATE:19180930T030000
RDATE:19190929T030000
RDATE:19201025T030000
RDATE:19211003T030000
RDATE:19221008T030000
RDATE:19230916T030000
RDATE:19240921T030000
RDATE:19251004T030000
RDATE:19261003T030000
RDATE:19271002T030000
RDATE:19281007T030000
RDATE:19291006T030000
RDATE:19301005T030000
RDATE:19311004T030000
RDATE:19321002T030000
RDATE:19331008T030000
RDATE:19341007T030000
RDATE:19351006T030000
RDATE:19361004T030000
RDATE:19371003T030000
RDATE:19381002T030000
RDATE:19391119T030000
RDATE:19451007T030000
RDATE:19461006T030000
RDATE:19471102T030000
RDATE:19481031T030000
RDATE:19491030T030000
RDATE:19501022T030000
RDATE:19511021T030000
RDATE:19521026T030000
RDATE:19531004T030000
RDATE:19541003T030000
RDATE:19551002T030000
RDATE:19561007T030000
RDATE:19571006T030000
RDATE:19581005T030000
RDATE:19591004T030000
RDATE:19601002T030000
RDATE:19611029T030000
RDATE:19621028T030000
RDATE:19631027T030000
RDATE:19641025T030000
RDATE:19651024T030000
RDATE:19661023T030000
RDATE:19671029T030000
RDATE:19711031T030000
RDATE:19721029T030000
RDATE:19731028T030000
RDATE:19741027T030000
RDATE:19751026T030000
RDATE:19761024T030000
RDATE:19771023T030000
RDATE:19781029T030000
RDATE:19791028T030000
RDATE:19801026T030000
RDATE:19811025T020000
RDATE:19821024T020000
RDATE:19831023T020000
RDATE:19841028T020000
RDATE:19851027T020000
RDATE:19861026T020000
RDATE:19871025T020000
RDATE:19881023T020000
RDATE:19891029T020000
RDATE:19901028T020000
RDATE:19911027T020000
RDATE:19921025T020000
RDATE:19931024T020000
RDATE:19941023T020000
RDATE:19951022T020000
END:STANDARD
BEGIN:DAYLIGHT
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
TZNAME:BDST
DTSTART:19410504T020000
RDATE:19410504T020000
RDATE:19420405T020000
RDATE:19430404T020000
RDATE:19440402T020000
RDATE:19450402T020000
RDATE:19470413T020000
END:DAYLIGHT
BEGIN:DAYLIGHT
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
TZNAME:BST
DTSTART:19410810T030000
RDATE:19410810T030000
RDATE:19420809T030000
RDATE:19430815T030000
RDATE:19440917T030000
RDATE:19450715T030000
RDATE:19470810T030000
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:+0100
TZOFFSETTO:+0100
TZNAME:BST
DTSTART:19681027T000000
RDATE:19681027T000000
END:STANDARD
BEGIN:STANDARD
TZOFFSETFROM:+0000
TZOFFSETTO:+0000
TZNAME:GMT
DTSTART:19960101T000000
RDATE:19960101T000000
END:STANDARD
END:VTIMEZONE
END:VCALENDAR
</pre>
<div class="moz-cite-prefix">On 2/10/20 06:58, Paul Smith wrote:<br>
</div>
<blockquote type="cite"
cite="mid:31fd456f-069d-739c-db79-dab8cedbb10f@pscs.co.uk">I have
a event with the following ICS source. My implementation of
recurrence rules is interpreting the timezone as changing to BST
on the first Sunday of March. It should be the last Sunday.
<br>
<br>
Most CalDAV clients seem to set the Timezone RRULE to
<br>
<br>
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
<br>
<br>
which is processed correctly, but this one just uses ...BYDAY=SU
which is being processed as 'every Sunday in March' (as described
in RFC 5545, section 3.3.10 ("If an integer modifier is not
present, it means all days of this type within the specified
frequency. For example, within a MONTHLY rule, MO represents all
Mondays within the month")
<br>
<br>
So, the recurrence event on 3rd March 2020 is at 11:50 UTC,
instead at 12:50 UTC.
<br>
<br>
Other CalDAV viewers I've tried (Mac Calendar, Mozilla Lightning)
seem to be handling it correctly, but I'm not sure if they're
processing the Timezone recurrence in a different way to my
implementation, or if they're just saying 'Hey, that's BST, I know
what BST is, so I'll ignore the Timezone definition in the event
and do it my own way'.
<br>
<br>
So, my question is: - should 'BYDAY=SU' be interpreted differently
in a Timezone definition than in other places (if so, where is
that described), or is the client which is generating this event
getting it wrong?
<br>
<br>
--------------
<br>
<br>
BEGIN:VCALENDAR
<br>
VERSION:2.0
<br>
PRODID:-//LigatureTech LLC.//SimpleCal 1.0.0//EN\r+
<br>
CALSCALE:GREGORIAN
<br>
BEGIN:VTIMEZONE
<br>
TZID:Europe/London
<br>
BEGIN:DAYLIGHT
<br>
TZNAME:BST
<br>
TZOFFSETFROM:0000
<br>
TZOFFSETTO:0100
<br>
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=SU
<br>
DTSTART:19700329T010000
<br>
END:DAYLIGHT
<br>
BEGIN:STANDARD
<br>
TZNAME:GMT
<br>
TZOFFSETFROM:0100
<br>
TZOFFSETTO:0000
<br>
RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=SU
<br>
DTSTART:19701025T020000
<br>
END:STANDARD
<br>
END:VTIMEZONE
<br>
BEGIN:VEVENT
<br>
UID:c8687991-80fa-4c2a-b273-3fb2f72a4c7c
<br>
SUMMARY:test
<br>
DTSTART;TZID=Europe/London:20200129T125000
<br>
DTEND;TZID=Europe/London:20200129T135000
<br>
RRULE:FREQ=DAILY
<br>
DTSTAMP:20200129T142840Z
<br>
CREATED:20200129T111403Z
<br>
LAST-MODIFIED:20200129T142840Z
<br>
SEQUENCE:1
<br>
TRANSP:OPAQUE
<br>
DESCRIPTION:test event 12
<br>
END:VEVENT
<br>
END:VCALENDAR
<br>
<br>
--------------
<br>
<br>
<br>
<br>
</blockquote>
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