[tz] Maritime time zones

[Tim Thornton]

Hi Zefram,
Thanks for that useful information.

Actually marine time zones DO use UT1, because of the link between time and
astro (or celestial) navigation.

The sign difference is because the conference that set up the nautical time
zones intentionally adopted this sign convention, which is unfortunately the
opposite of that used for terrestrial time zones - look up in any nautical
publication that has time zones, e.g. Admiralty Tide Tables or Admiralty
List of Radio Signals, and you will see this specifically maritime sign
convention used. I don't know which came first, the maritime or the time
zone sign convention, but it is confusing.

Tim



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-----Original Message-----
From: Zefram [mailto:zefram at fysh.org] 
Sent: 24 May 2012 16:23
To: Tim Thornton
Cc: tz at iana.org
Subject: Re: [tz] Maritime time zones

Tim Thornton wrote:
>Also, it uses GMT (i.e. UT1, not UTC) - I'm not sure if the tz database 
>can cope with this,

The tz database is effectively silent on which flavour of UT is the basis
for timezone offsets.  Its offsets are not meaningful at a subsecond level.
The maritime timezone system also long predates the need to distinguish
flavours of UT, and I'd be surprised if it really is specified to use UT1
rather than UTC.  So, in any case, the tz database can represent the
maritime timezones just fine.

>So they could be included in the TZ database

They are.  See the zones Etc/GMT-12 to Etc/GMT+12, in the "etcetera"
source file.  (There are also a couple more fixed-offset zones that are
outside the maritime system.)  Beware that the sign in the zone's name is
the opposite of the offset's sign, due to an historical confusion with the
negated-sign aspect of POSIX TZ format.

The initialisms of these zones match the names, and do not use the maritime
letters.  A single-letter initialism would be a problem, because the POSIX
TZ format requires initialisms to be at least three characters, so a
time-using program might reasonably expect the initialism to be at least
three characters.  The Theory file has a note about sticking to this.

However, aside from the historical fact of them already being in the
database, fixed-offset zones are generally out of scope for the tz database.
In general if you want a fixed offset, and in particular if you want a fixed
offset with an initialism based on the nautical letters, I suggest that you
use an actual POSIX TZ value.  For example:

	TZ=seaA-1
	TZ=seaM-12
	TZ=seaN1
	TZ=seaY12
	TZ=seaZ0

-zefram