Zulu time is another name for Coordinated Universal Time (UTC), the 24-hour world reference time based on the 0° longitude (Greenwich) meridian. It is also written as “Z time,” because the letter Z marks the zero-offset time zone and “Zulu” is the radio phonetic word for Z; a time such as 1430Z means 14:30 UTC. Weather services, aviation and the military use it so observations and schedules share one clock worldwide.
Zulu time does not change for daylight saving time. To convert it to U.S. Eastern Time, subtract 5 hours during standard time or 4 hours during daylight saving time. Before 1972 this reference time was usually called Greenwich Mean Time (GMT).
Source: NOAA National Hurricane Center, “What is UTC or GMT Time?”
Image: Standard time zones of the world (outline based on a CIA map), public domain, via Wikimedia Commons.