Zulu time

With all the time zones in the world, pilots need a way to reference the current, past or future time, and not have to worry about what time zone they are in.

This is where Zulu Time comes into play. Because a pilot may cross several time zones during a flight, a standard time system has been adopted. It is called Universal Coordinated Time (UTC) and is referred to as Zulu time. UTC is the time at the 0° line of longitude which passes through Greenwich, England. All of the time zones around the world are based on this reference. It is presented in the local Military time at Greenwich, England

To convert to this time in the United States, a pilot should do the following:

For daylight saving time, 1 hour should be subtracted from the calculated times.

Taken from the Pilots handbook of aeronautical knowledge (FAA-8083-25)

In order to determine Zulu time, you must first convert the local time into military time:

To obtain Military time from local time:

After 12 PM, add or subtract 12
Before 12 PM, do nothing

Example:

2:00 pm: 2 + 12 = 14 or 1400 Military time

10:00 am = 1000 (ten hundred hours)
8:00 am = 0800 (zero eight hundred hours)

To obtain local time when Zulu time is known:
Zulu Time – Factor = Local Military time

To obtain Zulu time when local time is known:
Local Military Time + Factor = Zulu Time

Factor is the offset from GMT (Greenwich Mean Time). It can be obtained from the Airport / Facility Directory (A/FD)

For example, the Texas A/FD shows that the offset for Houston is -6(-5DT). This means that this time zone is 6 hours behind Greenwich, and when daylight saving time is active, it is 5 hours behind.