bug#34315: [PATCH] icalendar.el: DURATION fix + more robust timezone handling

[Thomas Plass]

Eli Zaretskii wrote at 19:45 on August 10, 2020:
: 
: As for the patch, I've looked at it again now, and I don't think it's
: specific to MS-Windows, is it?

Exactly, the patch has nothing to do with MS-Windows' DST handling
(though incidentally, it was developed while dealing with that issue).

It fixes general issues with icalendar.el's handling of timezones as
commonly used in ical data and the standard tzurl.org repo.  It is
also completely backwards-compatible as far as I can make out.

Here's the README for the testcases I provided to Ulf along with the patch:

  This ZIP contains a patch for icalendar.el 0.19 and a set of iCalendar test 
files.
  VTIMEZONE sections contained therein were retrieved from http://tzurl.org.
  
   - Europe_Berlin-20181103T201500_no_in-calendar_VTIMEZONE.ics
     no VTIMEZONE, so no TZID references for DTSTART/DTEND, contains
     non-standard property X-WR-TIMEZONE which is invisible to icalendar.el
  
   - Europe_Berlin-20181103T201500_in-calendar_VTIMEZONE_tzurl_standard.ics
     standard VTIMEZONE ("Outlook-style")
  
   - Europe_Berlin-20181103T201500_in-calendar_VTIMEZONE_tzurl_historical.ics
     comprehensive VTIMEZONE including historical records
  
   - America_Creston-20181103T121500_in-calendar_VTIMEZONE_tzurl_standard.ics
     standard ("Outlook-style") VTIMEZONE
  
   - 
America_Creston-20181103T121500_in-calendar_VTIMEZONE_tzurl_historical_RDATE.ics
     comprehensive VTIMEZONE including historical records which use RDATE, not 
RRULE