The documentation in the users API controller lists possible values for timezones by calling ActiveSupport::TimeZone.zones_map, however this method is private in Rails 5.0:

MethodError: private method `zones_map' called for ActiveSupport::TimeZone:Class
    /home/dcleal/code/foreman/foreman/app/controllers/api/v2/users_controller.rb:50:in `block (2 levels) in <class:UsersController>'

It should be replaced by a method that remains public.

Note the same list is obtained by the time_zone_select form helper, which lists timezones through .all.