Where is TZ coming from?
Do you thing this is exact?
I think the time zone (located in /var/local/system/tz) is set when you initialy register the Kindle (based on "Your Kindle Account", "Country Settings") and is possibly also updated when you later synchronize.
I think that because one can only set the time from the Kindle Settings, Device Time menu, or alternately from a JB device, using the date and hwclock commands, or from synchronizing with a time server using the ntpdate command.
BTW, you can display the date and time from the home screen by selecting the first entry of a search for ;date. My Kindle has the correct time zone -- albeit without proper daylight saving time support -- and I don't recall ever setting it myself.