CalDAV DTSTART/DTEND now use UTC (Z-suffix) instead of TZID local
time. Without a VTIMEZONE component, some CalDAV servers strip the
TZID on return, causing ical.js to read the times as UTC — shifting
every event 4 hours early and letting taken slots appear free.
Slot query range changed from ±6h around UTC midnight (36-hour window)
to 3AM–6AM UTC the next day (~27h) which covers the full ET business
day without pulling in afternoon events from the previous day.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
chris
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