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    Re: British Summer Time versus GMT
    From: Frank Reed
    Date: 2013 Dec 16, 13:08 -0800

    Gary, you wrote:
    "I don't think (for example) that 10 am July 2nd is the same time as 10 am July 3rd, does anybody else?"

    Let's imagine a GPS-based clock display (a smartphone app that we could easily write today). As we cross the exact boundaries of the nautical time zones, it adjusts the display. For example, as we arrive at 7.5° W of Greenwich, the time displayed might be "18:15:30 July 2" just before we cross that longitude and "19:15:31 July 2" just after. The hours have changed. The time is changed. Then consider the case in mid-Pacific. If your clock reads "10:23:15 AM July 2" just before you cross 180° longitude, and it reads "10:23:16 AM July 3" just after you cross 180° longitude, you could say EITHER that the time displayed does not change (except for the normal passage of time) while the date has jumped, OR you could say that the time displayed has jumped by 24 hours. Both work.

    NEXT. Here is what I wrote previously:
    "there are 25 zone descriptors. Of those, 23 are centered on 15° intervals of longitude starting with the band centered on the Prime Meridian, and each differs from its neighbor by one hour. Then there are two special half-zone descriptors straddling the (nautical) dateline which both keep the same time, 12 hours from Greenwich, but one day apart and counted either as +12 or -12 depending on whether you're east or west of 180° longitude."

    And THEN Gary, you replied:
    "There are 25 standard zone descriptions, there are 25 letter designations, the whole alphabet minus only the "J". Let's see, the alphabet has 26 letters so if you take away one that leaves 25 letters for the standard time zone designations."

    So fill me in: where are you disagreeing with what I wrote previously?? I completely AGREE with you that there are 25 zone descriptors. That is a factual matter, and it's a very good point. As you say, just count the letters. There remains the question of explaining why they exist. That's a matter of pedagogy --a moving target-- not facts.

    Gary, you added:
    "Why try to make something that is so simple, complicated with your description?"

    That's really interesting, Gary. What did you find "complicated" in my description?

    You also wrote:
    "Now the country of Kiribati has added two "non-standard" zone descriptions, +13 and +14 so that they could keep time with their closer neighbors to their west.)"

    No. I disagree with that. That confuses civil "standard time zones" with the "nautical time zones". There are many standard time zones around the globe with only a rough correspondence with the idealized time zones which the nautical time zones implement. Kiribati has not added to the list of nautical time zones. If you are using "zone descriptor" as a synonym for "offset for UT", then, yes, there are many of them that do not match the letter-based nautical time zone system. Kiribati is by no means the first. Over a BILLION people live with time offset from Greenwich by some hours plus thirty minutes. Standard time zones are created by governments for the convenience of economics, nationalism, central political authority, and other reasons.


    -FER

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