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    Re: Time zone by letter and DST
    From: Gary LaPook
    Date: 2007 Aug 20, 01:54 -0700
    Boy that's a good question that I had never thought about before. It could be asked in a different way. "Are  the lettered designators equivalent to 'zone descriptions'  or are they equivalent to 'named time zones'?" e.g. Is "SIERRA" another way of saying "+ 6 zone description"  or is it another way of saying "central time zone?" I believe the latter makes more sense. Part of the year clocks in the central time zone are set according to  zone description + 6 and during the summer, + 5. The reason it makes more sense to me that the letters are associated with the named time zones is that daylight savings is not universally kept in an entire time zone, with countries near the equator staying on standard time all year long as there is little change in the number of daylight hours during the year. For example in Brazil only certain areas of the country go on DST even though in the same time zone.

    Another argument is that the chart labeled "Standard Time Zones"   http://aa.usno.navy.mil/faq/docs/world_tzones.html

    notes that "Daylight savings time is kept in some places." It also gives the formula for finding standard time in each zone : "Standard time = Universal Time  + value from table" then gives the number of hours to add to UT to find standard time. Note, the sign given for the number of hours is the reverse of the sign for the "zone description." According to the table, to calculate SIERRA time, central standard time, you add (-6) to UT. Yet while keeping central standard time the zone description is + 6.

    Navy practice is to note in the log and next to the clocks on board the zone description. Army practice is to use the letter designations with date/time groups.

    gl


    Lu Abel wrote:
    Bill:
    
    Time zones can be pretty confusing, even without governments throwing in
    oddities (like India being 4h 30m ahead of GMT/UTC).  You've got several
    concepts thrown together in your note:
    
    To start, let's remember that the whole idea of having local time zones
    is simply to have noontime be somewhere around when the sun is on our
    local meridian.
    
    To that end, the world is divided into 24 time zones in 15 degree
    increments outwards from Greenwich.   Actually, it's 25 zones, there are
    two 7.5 degree zones on either side of the International Date line.
    Greenwich is zone Z (Zulu in the international phonetic alphabet). Zones
    are lettered from A upwards going eastwards from Greenwich to the IDL,
    ending in "M" as the 7.5 degree zone just west of the IDL.  Then they
    are lettered from N upwards going westwards from Greenwich.   These are
    bands of local STANDARD time we might keep at sea.
    
    But then the governments intervene.  Land time zones are drawn for
    convenience of commerce, not necessarily to place the sun in the correct
    position in the sky.   In the US the effect is to have most of our time
    zones end considerably west of their proper meridian.  For example, the
    Eastern time zone extends to about 87 degrees west longitude, despite
    the fact that global time zone Romeo, on which it is based, stops at
    82.5 degrees.   The Central, Mountain, and Pacific time zones are of
    normal width, but displaced to the west by the Eastern zone.
    
    Things are often even crazier in other parts of the world:  China spans
    four time zones (E through H) with its northeastern tip extending into
    the I time zone, but it keeps but a single time throughout the country,
    H time (which means the sun comes up REALLY late in western China).
    
    I mentioned India above.  India spans two time zones, E (4 hours ahead
    of GMT) and F (five hours ahead of GMT).   Perhaps as a compromise to
    allow the whole country to be on a single time zone, clocks in India are
    4h 30m ahead of GMT.
    
    You can find a nice map of 15 degree time zones and country time zones
    at http://aa.usno.navy.mil/faq/docs/world_tzones.html
    
    Then there's Daylight Savings Time (aka Summer Time east of the
    Atlantic).  People would rather have extra daylight in the evening than
    in the morning, so by government fiat we all set our clocks ahead during
    the summer.  Clocks in the eastern US are no longer five hours behind
    GMT but only four hours behind GMT.
    
    Your main question was how to record time in logs.
    
    I don't think there's a universal practice.   For ships at sea, one
    tradition is to log in local (standard) time -- and clearly note in the
    log when shifting time zones.  In this tradition the ship's time zone is
    updated on the top of the next hour after crossing the dividing
    meridian.  The DR for that hour is annotated with the time for both the
    old and new time zones.   Time zone letters are not used, but rather
    Zone Designator offsets (the amount in hours which must be added to
    local time to obtain GMT).   Thus, for example, the a DR after crossing
    a time zone divider meridian might be labeled 1000 ZD+4 / 1100 ZD+3,
    showing the local time to be 1000 with a four-hour offset to GMT or 1100
    with a three-hour offset.
    
    I think that logs and logging are highly personal, as long as what's
    going on is clear enough so someone else can read a log.  I know of some
    people who log everything in GMT.   No time zone conversions needed.
    
    One thing I would advise against is using the letter designators as
    opposed to using a zone designator (eg, "ZD+4" or just "+4").  That
    removes any possible ambiguity around whether the zone letter (eg, "R")
    needs to be adjusted for daylight time.
    
    Hope this helps.
    
    Lu Abel
    
    Bill wrote:
      
    The basic concept is clear to me, it is usage that I cannot track down.
    
    For example 0700 UTC/GMT would also be 0700 Zulu.
    
    Here in Indiana, USA I am in the eastern time zone at my home and on
    portions of Lake Michigan. (Indiana time zones is its own amusing topic.)
    
    When on standard time I subtract 5 hours from UTC for zone time.  When on
    daylight saving time I subtract 4 hours.
    
    Therefore if 0700 UTC or Zulu, my zone time is either 0200 or 0300,
    depending on whether or not daylight saving time is in effect.
    
    The designator for my time zone is Romeo (R). If I attach R or Romeo to the
    time in comminations or in the log, how do I account for standard or DST?
    Or is it just assumed that when used with a letter it is always standard
    time?
    
    Bill
    
    
    
        
    
    
    
      


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