My timezone problems are fixed...
Thanks to Jaidev and Brucer for their suggestions.
I have now fixed the time on my TiVo. Contrary to the detail I gave previously, I HAD broken my postcodezones file. I imagine this happened on ftp (doubt it) or edit (windows and UNIX end of line differences, I should have seen that days ago) - the file looked OK and fixtivotime displayed a postcode match, but what it matched to wasn't Pacific/Auckland it was Pacific/Auckland<invisible character>, so I got no offset or NZST timezone from the Auckland file.
So now when I run fixtivotime I get exactly what pendantic me wants:
(and when I reboot, or cron runs to sync time, or I do guide data update... it's stays correct)
[TiVo [p0] ~]# fixtivotime
Our postcode is 02113. Found matching timezone Pacific/Auckland
Offset 43200 for timezone NZST, daylight saving is 0
Connecting to time server 207.126.98.204
t was 3301642873 from server
t is now 1092697273
Time is 2004-08-16 23:01:13
Time set to: Mon Aug 16 23:01:13 2004
Have a nice day.
Time set to: Mon Aug 16 23:01:13 2004
Have a nice day.
Why I am writing all this you ask. Because it'll probably happen to someone else (maybe).
:)