Cannot set events with a user preferred time zone

[stevepiercy]

It is not possible to create local events with a local time zone in plone.org or in the Volto demo, but I can in the Classic UI demo.

Classic UI Demo behavior

• Login to https://classic.demo.plone.org/en as admin/admin.
• Go to Date and Time control panel.
• Observe the only available time zone is Europe/Berlin by default.
• Add any time zone, such as America/Los_Angeles, and click Save.
• Click Home to return home.
• Create a new event. The event inherits the site's default time zone.
• Go to User Preferences, select the available time zone, America/Los_Angeles, and click Save.

Plone.org and Volto Demo behavior

• Repeat all the steps above. Observe that the User cannot set their preferences for a time zone, and must therefore rely upon the site preference for a time zone when creating events.

I don't know whether this is a bug in Volto or some other dependency.