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Nope, other than the name of an African tribe, Zulu has nothing to do with a ‘folk’ expression. It’s part of the acrophonic alphabet internationally adopted for use in communications by all international organizations, such as the ITU, NATO, ICAO, FAA, etc. It’s the same as Alpha=A, Bravo=B, Charlie=C, Delta=D, Echo=E, etc. In radio communications we often get static or other interference which can foul the understanding of a word or words. Spelling it out a letter at a time can also be misinterpreted. But if you use the international acrophonic word for each letter, you have a better chance of receiving and understanding the message.
Zulu=Z. Z is the letter designation for zero. The 0 Meridian is the Prime Meridian. The longitudinal part of Earth coordinates is measured in degrees (and minutes and seconds) East or West of the Prime Meridian. Time zones, political borders not withstanding, are 15 degrees, or one hour of time (360 deg/24 hrs=15 deg/hour) +(East) or -(West) of the Prime Meridian.
Looking down at the Earth from directly above the North Pole, it turns counter-clockwise. If the Prime Meridian passes through a direct line from the Sun at High Noon, and New York is in a time zone 5 time zones away, subtract 5 hours from 12:00. It’s 7:00am in New York.
Without these standardized references, we’d have a very tough time communicating with each other around the globe.
“I’ll call you back at 4:30”
So what do you mean by 4:30? AM or PM? Or are you using 24-hour time? If you’re in Los Angeles, what would someone in Australia think you mean? What would someone in an airliner over the Atlantic think you mean? Will the person get your call?
If you and the other person both keep an extra clock running Zulu time, no problem. You’d say: “I’ll call you back at 04:30 Zulu.” It means 24-hour clock. IOW, the other person would expect your call correctly … it would be 04:30Z, the same as yours.
As a side note: In Greenwich it would be 4:30 in the morning.