App claims not to know user's timezone

[dreeves]

### Desiderata
- [ ] Discuss how impactful this is, does it replicate for us?
- [ ] TBD

A user reports that their timezone is listed in BeemiOS settings as "Unknown" for some unknown reason:

Cognata

• https://secure.helpscout.net/conversation/2112981500/141009?folderId=2972058

• 282

• 31

• https://github.com/beeminder/BeeSwift/blob/master/BeeSwift/SettingsViewController.swift#L117

• 361

Verbata: timezone, time zone,