There's a file called /etc/timezone that I believe has that info. All of the zone info is in /usr/share/zoneinfo or some such directory. Get Debian's latest source:http://packages.debian.org/source/sid/tzdata
And copy the zoneinfo out of it, then copy it to /etc/localtime. After that (and likely a reboot), you SHOULD be good.
Note that I haven't tried this.