Under some circumstances, Team Calendars will need to locate a timezone (such as during an import of an .ics file). If the file does not contain a valid timezone, TC will attempt to obtain the system's timezone in this order:

1. atlassian.confluence.settings/defaultTimeZoneId in Confluence's bandana table.
2. user.timezone Java sysprop, if set.
3. Platform-specific default timezone id. JRE identifies this differently in MacOs, Linux and Windows.
4. The default value: GMT

However, some of the valid values for Java's user.timezone such as "UTC" or "GMT" are not considered valid for Team Calendars, since TC's supported timezone values are based on an intersection of Joda and Ical4j timezones: teamcal_supported_tz.txt

Because TC does not consider values like "UTC" to be valid, this can cause problems during certain scenarios, like the import calendar problem described in CONFSERVER-57756