Steps to reproduce:

1. Click on the user profile icon, select "Personal settings".
2. In the "Your timezone" section select "GMT Offset" and then one of the options, such as "GMT-12".
3. Press "Save changes".

A "Couldn't save your preferences" notification appears.

When request is sent, the JSON looks like

{{

{"timezone":"GMT Offset/GMT+12","locale":"en_US","notify":false,"userNotificationsMimeType":"html"}

}}

The timezone attribute should probably be "Etc/GMT+12".