"Zulu" time is that which you might 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 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, southeast of central London. As a result, when the concept of time zones was introduced, the "starting" point for calculating the different time zones was/is at the Royal Greenwich Observatory. When it is noon at the observatory, it is five hours earlier (under Standard Time) in Washington, D.C.; six hours earlier in Chicago; seven hours
earlier in Denver; and, eight hours earlier in Los Angeles.
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. The U.S. time zones are Eastern ["R", "Romeo]; Central ["S", "Sierra"]; Mountain ["T", "Tango"]; Pacific ["U", "Uniform"]; Alaska ["V", "Victor"], and Hawaii ["W", "Whiskey"].