Alarm trigger doesn't consider timezone

Note: Not sure where to post this issue, since I'm not sure on how to understand RFCs.

Steps to reproduce

• Be on another TZ than UTC (calendar must have this TZ)
• Add a full day event
• Add a reminder "on the day of the event" at a certain time, say 17:30 here

Result

DTSTART;VALUE=DATE:20200130
DTEND;VALUE=DATE:20200202
[⋅⋅⋅]
BEGIN:VALARM
ACTION:EMAIL
TRIGGER;RELATED=START:PT17H30M
END:VALARM

This will be interpreted by server as 17H30 UTC, not in user's local timezone. Since it's a full day event, the event DTSTART hasn't got any timezone information, and sabre/object doesn't seem to consider the calendar's VTIMEZONE to get effective trigger time.

RFC 5545 says

The "TRIGGER" property specifies a duration prior to the start of an event or a to-do. The "TRIGGER" edge may be explicitly set to be relative to the "START" or "END" of the event or to-do with the "RELATED" parameter of the "TRIGGER" property. The "TRIGGER" property value type can alternatively be set to an absolute calendar date with UTC time.

So…I'm guessing it's 17:30 relative to an UTC date after all, but it's quite odd for users to expect this. Should we only work with absolute dates instead ?

Evolution and Thunderbird Lightning don't really offer such a functionality. And Google Calendar only has gives TRIGGER:P0DT17H30M0S for the same parameters…

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Start by reproducing the timezone difference with the supplied ICS event and alarm while using a non-UTC calendar timezone. Read the RFC 5545 TRIGGER rules and inspect how sabre/object interprets the date-only DTSTART and calendar VTIMEZONE; done means the expected behavior is decided and the alarm trigger is handled consistently with that decision.