Aviation is a global business with aircraft constantly changing time zones, so it only works if we all use the same point of reference when talking about the time!
Aviation uses 24-hour time based on the standard Universal Coordinated Time (UTC). This is also known as Greenwich Mean Time (GMT) and in aviation is referred to as Zulu Time (Z).
For example, 7pm UTC is referred to as 1900Z (pronounced as “nineteen-hundred zulu”).
This UTC (or ‘zulu’) time is based on London winter time (i.e. it does not change with daylight saving time).
Local Mean Time (LMT) adjusts the Zulu Time for a given position east or west of the prime meridian (also called the Greenwich meridian). Travelling east or west of the prime meridian puts us on a different line of longitude, and all points along a line of longitude will have the same local mean time.
The Earth rotates once per day (24 hours), so we know that 360° = 24 hours. We can then divide this into smaller changes of longitude:
If we travel east, we add time and if we travel west, we subtract time. We can use this reference point to calculate the local mean time based on our longitude.
What is the LMT in Sydney, Australia (33° 52′ S, 151° 13’E) when the UTC time is 0730?
1 Convert the degrees east/west (151°) to its equivalent change in time
1° = 4 minutes
151° x 4 = 604 minutes = 10 Hours 4 Minutes
2 Convert the minutes east/west (13′) to its equivalent change in time
1′ = 4 seconds
13′ x 4 = 52 seconds
We can round 52 seconds up to 1 minute
3 Add the two together to get the total change in time
10 Hours 4 Minutes + 1 Minute = 10 Hours 5 Minutes (1005)
4Add the time to UTC if it is East
Minus the time from UTC if it is West
Sydney is 151° 13’E so we will add the time
UTC was 0730
0730 + 1005 = 1735 LMT