[tz] Maritime time zones

Tim Parenti tim at timtimeonline.com
Fri May 25 02:24:11 UTC 2012

On 24 May 2012 17:18, Alan Mintz <Alan_Mintz+TZ_IANA at earthlink.net> wrote:

> Note that the nautical date line is different from the international date
>> line.
> Where (as of 2010)?

The "nautical date line," used when not in territorial waters, is the 180th
meridian by convention, which splits the 15-degree range from 172.5°E to
172.5°W into two 7.5-degree ranges: One for GMT+12 and one for GMT–12.
 Obviously, when one is in the territorial waters of a nation, though, one
should set clocks to local jurisdictional time instead.

The "international date line," on the other hand, is simply a line drawn
for convenience to show the "locations" of time transitions on the order of
24 hours.  The line is itself not directly legislated by any governing
body, nor does it have any meaning intrinsic to itself, except to
distinguish where a roughly day-long disparity exists between adjoining
"time zones" (whether nautical or jurisdictional).  Regardless of their
geographical location, nations can "move" this line so as to put themselves
on one side or the other simply by making their own UTC offset positive or
negative.  As such, crossing into territorial waters could mean a clock
shift of more than 24 hours; for instance, if one were to enter the UTC+14
portion of Kiribati from international waters which are geographically in
the GMT–12 zone (ignoring, for now, the minor differences between the two
timescales), one would shift clocks ahead by 26 hours.

This is why the international date line is so jagged.  Where appropriate,
it is equivalent to the nautical date line on the 180th meridian.
 Elsewhere, it veers to either side to follow the appropriate boundaries of
territorial waters, serving only to separate "date changes" (changes of
about 24 hours) as a visual cue on a map to travellers.  Since the
boundaries of territorial waters are often hard to pin down precisely,
different cartographers will tend to draw these boundaries on maps slightly
differently, often with clean diagonal line segments to be aesthetically
pleasing, as in
 This does not mean these diagonal lines represent the actual geographical
and logical boundaries where "date changes" occur; rather, the actual
international date line is the result of the sum total of all the nearby
nations' time zone legislation.

Theoretically, there could even be geographical "pockets" where
negative-offset nautical time is used, surrounded completely by
positive-offset jurisdictional time (or, conversely, positive-offset
nautical time surrounded by negative-offset jurisdictional time).  Such a
scenario would mean the international date line wouldn't actually be a line
at all.  (I do not know whether such a scenario actually exists at present,
nor do I believe it would be an easy determination to find out.)
 Technically, a vessel travelling straight through such a region should
have several "date changes" back and forth; however, a reasonable person
travelling through such a region would probably just choose to "stay on one
day or the other" as they found it convenient (keeping in mind the
potential for disparity amongst others who may have made the inverse
decision), until reaching a region where the date doesn't keep changing on
them anymore. *;)*

Tim Parenti
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