[CentOS] Where is timezone string stored?

I am having problems with RoundCube:

'Your session is invalid or expired'

So I went looking for logs and in /var/log/roundcube/errors I find LOTS 
of warnings about problems with my timezone.  Kind of a challenge to 
copy the log entries over here (will do if needed).

Anyway, for right now I am looking as to where my 'Detroit 
American/New_York' (what I am seeing in Gnomes calendar preferences) 
string is stored.

Roundcube seems to want 'America/New_York'?

Shouldn't I be seeing 'America/Detroit' when I look at the calendar 
selection in Gnome?