Converting UTC to local time
The current time is: 10:11:17 UTC.
The current time is: 03:11:17 or 03:11:17 am in California.
The current time is: 04:11:17 or 04:11:17 am in Denver.
The current time is: 05:11:17 or 05:11:17 am in Chicago.
The current time is: 06:11:17 or 06:11:17 am in New York.
Pilots and astronomers are two professions that use a standard time based on the time in Greenwich England, the home of the Royal Observatory and the prime meridian. Since 1884 the time in Greenwich was used as a standard and referred to as Greenwich Mean Time (GMT). The time zone at Greenwich was assigned the letter Z and so GMT is often referred to as Zulu time and shown with the letter Z. In 1928 Coordinated Universal Time (abbreviated UTC) was introduced by the International Astronomical Union to more precisely measure time. Details are at the US Naval Observatory
Pilots often fly across multiple time zones in a single day and to avoid confusion use UTC time for filing flight plans, accessing weather, and calculating time en route. They need to be able to convert UTC time to local time and vice versa. The conversion from UTC to local is a two step process. First, subtract the appropriate offset from UTC and then covert to AM or PM. For example, if the time is 2300Z and daylight saving time is not in effect, the time in New York (Eastern Time Zone) is 2300 - 500 = 1800 or 6 PM.
It gets a little more complicated if the time is 0200Z. Subtracting 5 from 2 gives negative 3. Then you need to subtract 3 from 12 to get 9 PM. I usually find that it is easier to add 12 first, then subtract. So in this case add 12 plus 2 to get 14 and then subtract 5 to get 9 PM.
Also, for me, subtracting 7, 8, 9 is a bit difficult to do. It is easier for me to subtract 10 and then add 3, 2, or 1. So in the winter in California I’d convert 2300Z by subtracting 10 and adding 2: 23-10 = 13 + 2 = 15 or 3 PM. I’d do the second example the same way, 0200Z is 26 -10 = 16 + 2 = 18 or 6 PM.As I write this it is 0120Z. To convert to PDT I’d add 12 to get 1320, then subtract 10 to get 0320 and add 3 to get 06:20 PM.