When checking current shifts to trigger slack notifications, a stable
value for timezone should be used (to avoid triggering multiple
duplicated notifications; using the first `VTIMEZONE` value available
may change on refresh, besides being potentially incorrect).
Also, `VTIMEZONE` shouldn't be used as the calendar timezone, it just
describes a [timezone
definition](https://icalendar.org/iCalendar-RFC-5545/3-6-5-time-zone-component.html)
which can later be [used when specifying a start/end
date/datetime](https://icalendar.org/iCalendar-RFC-5545/3-2-19-time-zone-identifier.html)
("The parameter MUST be specified on properties with a DATE-TIME value
if the DATE-TIME is not either a UTC or a "floating" time.").
OTOH, Google uses the non-standard[ `X-WR-TIMEZONE`
](https://github.com/collective/icalendar/issues/343)field as fallback
TZ for date-times without a `TZID`. Try to use this when available,
otherwise fallback to `UTC`.
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