A4a - Zulu Grenwich Mean Time

[4] – Zulu – GMT - Chronology Love Field, Dallas – 86972 – 26000 –Andrews

There are a number of different time references on the Air Force One Radio Transmission Tapes and the Andrews Log.

Events as they happened in Dallas are in Central Standard Time (CST), which is one hour difference from Washington, D.C., which is on Eastern Standard Time (EST), while the pilots, military personnel and radio operators used 24 hour military style and Zulu time, which is Grenwich Mean Time (GMT) in England, five hours earlier than EST.

Zulu Time

What is "Zulu" time?
"Zulu" time is that which is more commonly know as "GMT" (Greenwich Mean Time). Our natural concept of time is linked to the rotation of the earth and we define the length of the day as the 24 hours it takes (on average) the earth to spin once on its axis.

As time pieces became more accurate and communication became global, there needed to be a point from which all other world times were based. Since Great Britain was the world's foremost maritime power when the concept of latitude and longitude came to be, the starting point for designating longitude was the "prime meridian" which is zero degrees and runs through the Royal Greenwich Observatory, in Greenwich, England. When the concept of time zones was introduced, the "starting" point for calculating the different time zones was agreed to be the Royal Greenwich Observatory.
Unfortunately the Earth does not rotate at exactly a constant rate. Due to various scientific reasons and increased accuracy in measuring the earth's rotation, a new timescale, called Coordinated Universal Time (UTC), has been adopted and replaces the term GMT.

The Navy, as well as civil aviation, uses the letter "Z" (phonetically "Zulu") to refer to the time at the prime meridian. NOAA satellites use Zulu Time or Coordinated Universal Time (UTC) as their time reference. The satellite images that appear on NOAA's Web sites are stamped in Zulu time. The Department of the Navy serves as the United States official timekeeper, with the Master Clock facility at the U.S. Naval Observatory, Washington, D.C.

http://wwp.greenwichmeantime.com/what-is-gmt/index.htm

Greenwich, England has been the home of Greenwich Mean Time (GMT) since 1884. GMT is sometimes called Greenwich Meridian Time because it is measured from the Greenwich Meridian Line at the Royal Observatory in Greenwich. Greenwich is the place from where all time zones are measured.
Check out the GMT timestamp to see how accurate your computer time is.
The Greenwich Meridian (Prime Meridian or Longitude Zero degrees) marks the starting point of every time zone in the World. GMT is Greenwich Mean (or Meridian) Time is the mean (average) time that the earth takes to rotate from noon-to-noon.

GMT is World Time and the basis of every world time zone which sets the time of day and is at the center of the time zone map. GMT sets current time or official time around the globe. Most time changes are measured by GMT. Although GMT has been replaced by atomic time (UTC) it is still widely regarded as the correct time for every international time zone.
Where is Greenwich, England?

Longitude 0° 0' 0"
Latitude 51° 28' 38"N (North of the Equator)

Greenwich Mean Time is international time, the basis of the world time clock. GMT marks precision time and military time (sometimes called Zulu Time). Defines date and time and the exact time. The atomic time clock is adjusted by leap seconds to maintain synchronicity with GMT.
http://wwp.greenwichmeantime.com/what-is-gmt/index.htm