Difference between zone time and local sidereal time? What is the key difference between LST and ZT?
Also, are UT, GMT and Greenwich Mean Solar Time the same?
Universal Time (UT) and Greenwich Mean Time (GMT) are based on the average (mean) position of the sun. The mean position of the Sun crosses the meridian every 24 hours. There are subtle differences between UT and GMT, but they are the same for most practical purposes. (The Wikipedia article for Greenwich Mean Time points out that the start of the day under GMT changed from noon to midnight sometime in the past. The Time and Date.com article GMT versus UTC describes GMT as a time zone and UTC as a time standard.)

Since our clocks are also based on the mean position of the Sun, the time in your time zone is a fixed number of hours before or after UT.

Local sidereal time (LST) is the time according to the stars at your longitude. Let's assume that star X is on the meridian at 0 hours (midnight). The same star will return to the meridian after 23 hours 56 minutes 4 seconds (approximately) of clock time. In other words, the star time runs almost 4 minutes fast each day. Therefore, there are two key differences between sidereal time and the clock time:


Sidereal time is different at each longitude. Clock time is the same for all longitudes within a time zone.
To convert from clock time to sidereal time, you need to know the time and date.