Problem

Quite a number of timezone disappears from the timezone drop down menu (the Create Calendar dialog box):

For the example above, it only shows timezones from America and Africa

Diagnosis

Diagnostic Steps

• Make sure that you try this on multiple browsers to make sure it is not a client-side issue
• Try to enter safe mode, in Confluence Admin >> Manage Add-Ons, and enable only Team Calendars for Confluence

Cause

Cause unknown, though it may most likely be due to corrupted caches

Resolution

Restarting Confluence seems to resolve the issue completely. You can also try to simply flush all the caches in Confluence Admin >> Cache Statistics.