For many years seamen and navigators have been referring to Greenwich Mean Time (GMT) as "Z-time". GMT has been noted as Greenwich Civil Time (GCT) and lately with the advent of a universal community as Universal Coordinated Time (UTC). Note that the initials UTC do not fit the words Universal Coordinated Time. This is because the United Nations, still considering French as the international language, have designated the official designator for Universal Coordinated Time as UTC, as the initials would appear in French.
GMT or Z-time has been used for centuries by seamen and navigators in referring to the time at 0° longitude (through Greenwich, England...at the Prime Meridian). Z-time is noted in many navigation texts including texts like "AIR NAVIGATION" by Weems, and also in most of the navigation tables published by the United States Naval Observatory and Her Majesty's Stationery Office. These tables used by modern day navigators include such publications as: THE AIR ALMANAC, NAUTICAL TABLES AND EPHEMERIS, and numerous navigation tables published by the U.S. Hydrographic Office as "H.O. TABLES".
One thing is sure...whether the time is noted as GMT, GCT, UTC or Z it all relates to the time at 0 degrees longitude (at the prime meridian).
The question now becomes, how and why do we refer to GMT, GCT, UTC, etc, as Z-time? After the end of the U.S. Revolutionary War, an American sea captain named Nathaniel Bowditch was quite perturbed that all honor and recognition of seamanship and celestial navigation was allocated to British sea Captains, due to the high visibility and respect for the British Royal Navy. Bowditch proclaimed that American sea Captains were just as proficient and capable as their English counterparts. Therefore; he decided to write a book to emphasize the fact. In the late 1700's, Bowditch wrote his now famous navigation textbook, "THE AMERICAN PRACTICAL NAVIGATOR". This book was proclaimed throughout the seafaring world as one of the best textbooks in sea-surface celestial navigation that was ever written. This text was so well written that it is still referred to today as a navigation textbook at The U.S. Naval Academy, The U.S. Coast Guard Academy ,and the U.S. Merchant Marine Academy.
In his book, Bowditch realized that a method of noting local time zones was needed for references in a ships log throughout the world. It must be remembered that this was before the times of Eastern Standard Time, Pacific Standard Time, or even notations like "Daylight Savings Time".
It had been well understood by navigators that there being 24 hours in a solar day (23h56m 4.091s in a sidereal day), and since there are 360° of longitude around the earth, that each 15° of longitude constituted another 1 hour time-zone. What Bowditch proposed in THE AMERICAN PRACTICAL NAVIGATOR was that the prime meridian (0° longitude) be designated as the center of a 15° time zone, i.e. 7.5° each side of 0° longitude. He then divided the remaining longitudes into 15° segments, i.e. 7.5° each side of 15°East longitude, 7.5° each side of 30° East longitude, 7.5° each side of 45° East longitude, etc, etc, around the world. He then designated a letter of the English alphabet to each time zone starting with the first zone east of the Prime Meridian. This made the zone centered on 15° East longitude, time zone "A", the time zone centered on30° East longitude, time zone "B", and so on. When he came to the International Date Line at 180° longitude, realizing that although the time was the same in each 7.5°segment each side of 180° longitude...they were different days. Therefore as the International Date Line was approached from the west, Bowditch designated the section from172.5° East longitude to 180° longitude as time zone "M". Bowditch then proceeded with the times zones west of the Prime Meridian Time Zone. The first zone centered on 15° West longitude became time zone "N", the next time zone west centered on 30° West longitude became time zone "O", etc, etc. Approaching the International Date Line from the east, Bowditch designated the remaining "halfzone" from 172.5° West longitude to 180° longitude as time zone "Y". This left the letter "Z" remaining and this was reserved for the Prime Meridian Time Zone through Greenwich, England. Therefore GMT, GCT, UTC, etc, is called "Z-time". This notation, "Z", is appended to times on most navigational, meteorological, and astronomical charts today to indicate that the time of the chart is in "Greenwich Mean Time".
This leaves us with the dilemma...there are 24 hours in the day...and there are 25 time zones (since each half zone either side of the International Date Line became a designated zone). Therefore 25 letters of the English alphabet were used in Bowditch's time zone notations. What letter was left out?
Since the sound for the letter "J" was frequently not found in many languages of the time, Bowditch left out the letter "J".
Since well before the American Revolution seamen have been using a phonetic alphabet to indicate letters of the alphabet. Today the commonly used phonetic alphabet for A, B, C, D, etc, is ALPHA, BRAVO, COCOA, DELTA, etc. During the period from WW I through WW II the commonly used phonetic alphabet for A, B, C, D, etc, was ABLE, BAKER, CHARLIE, DOG, etc. Even Police Officers today use a phonetic alphabet when describing names and license numbers so there won't be any misunderstanding. How many times on TV shows like RESCUE911, COPS, DRAGNET, etc has a police officer said something like "Adam - Item -Sam" to indicate the letters A-I-S? Since the early days of English seafarers a phonetic alphabet has been in use. U.S. NAVY Signalmen have used such phonetic designations for over 200 years. The letter "J" (missing from Bowditchs time zone designations) has been spoken as "JIG" for over 200 years. Now for the trivia of the day. The "original" expression "In jig time" was a sailors expression, meaning "in no time at all", because there is no "JIG" time zone.
In summary, Greenwich Mean Time is usually written today, in most technical writings, using a 24-hour clock notation, and appended with the notation "UTC", "GMT", "GCT", or "Z". In most handwritten and non-technical writings, the time is usually just appended with the notation "Z". When spoken, any of the above notations are appended to the time, including the various phonetic words to indicate the letter "Z", such as "ZEBRA", "ZULU", or "ZED", etc.
Universal Coordinated Time (UTC) is the time at zero or reference meridian. Time changes one hour with each change of 15 degrees in longitude. The five time zones in the US proper and Canada roughly follow these lines.
0000* and 2400* interchangeable. (2400 is associated with the date of the day ending, 0000 with the day just starting.)