This article in particular is good about describing this:

https://web.archive.org/web/20140221080916/http://www.cstdbill.com/tzdb/tz-how-to.html


It's too bad that we have to use archive.org to get at this seemingly fundamental doc.  Any way we could host it along side tz-link.html in the repo?

Date: Mon, 19 Oct 2015 13:55:24 -0400
From: tim@timtimeonline.com
To: Jim.Marvel@honeywell.com
CC: tz@iana.org
Subject: Re: [tz] 2015g: Is the 1:00 under Pacific/Norfolk "Rules" a typo?


On 19 October 2015 at 10:06, Marvel, Jim (CPCOE) <Jim.Marvel@honeywell.com> wrote:
Hello,
       Is the 1:00 under Pacific/Norfolk "Rules" a typo?
No; the lines

                     11:30  -      NFT    1974 Oct 27 02:00 # Norfolk T.
                     11:30  1:00   NFST   1975 Mar  2 02:00
indicate that, between 1974-10-27 02:00 local and 1975-03-02 02:00 local, Pacific/Norfolk observed Daylight Saving in the amount of one hour on top of its normal offset of UTC+11:30; that is, the area was on UTC+12:30 for that period.

This was introduced recently and is documented accordingly:

# From Paul Eggert (2015-09-23):
# Transitions before 2015 are from timeanddate.com, which consulted
# the Norfolk Island Museum and the Australian Bureau of Meteorology's
# Norfolk Island station, and found no record of Norfolk observing DST
# other than in 1974/5. See:
# http://www.timeanddate.com/time/australia/norfolk-island.html

--
Tim Parenti