Instance users configured with any of the Etc/GMT timezones see incorrect date/time values.
Steps to Reproduce
1) Add the Etc/GMT timezones to make them selectable in the instance.
2) Enable the timezone picker or put one user in each of the following timezones: US/Pacific, Europe/Brussels, Etc/GMT-8.
3) Switch timezone to Europe/Brussels or log in as your Europe/Brussels user.
4) Create an incident. Note the open/create time.
5) Switch timezones to US/Pacific or log in as your US/Pacific user.
6) View the incident from step 4. The open/create time should be 9 hours behind the Brussels user (e.g., if brussels opened at 23:59:59, US/Pacific should see 14:59:59).
7) Switch timezones to Etc/GMT-8 or log in as your Etc/GMT-8 user.
8) View the incident from step 4 and note the open/create times. They should display the same as those from step 6, but they do not.
The problem has been fixed since Geneva, so there is no Etc/GMT related timezones anymore in the available slushbucket, once you right click Time Zone label and select "Configure Choices" in Geneva and later versions.
However customers can still add those Etc related timezones manually without selecting from the slushbucket list. In this case, users with those timezone settings still see incorrect date/time values on later versions.
Etc/GMT are specific timezones that conform with POSIX that are reversed compared to GMT. For example, Etc/GMT-8 is actually GMT+8. The platform works as expected, and Etc/GMT related timezones are not supported. Please choose city/area-based timezones from the available list.
Related Problem: PRB623606