I want to clarify what is happening on the event announcement message in Slack:
This actually uses a Slack date component that displays the time based on each user's personal timezone. Thus each user will see the time differently, which is pretty nifty. Although Slack currently doesn't support displaying the timezone in the date.
If we did not use this component, it would force a single time + timezone to be displayed for all users.
We could put in a feature request that Slack adds a timezone display in their date component, so every user sees their personal timezone. Would that, or the hard-coded timezone for all users, be preferable?