I think the base time BT is the time on the naval base, so the Zone Time ZT.
Example:
1941 December 10 - base time 17-15-00
The naval base is Pearl Harbor on 21°20.7'N and 157°58.5'W and is located in the Zone Time GMT-10
Determine Greenwich Mean Time GMT and Local Mean Time LMT on naval base Pearl Harbor
Step 1 - GMT = BT + 10
BT 17-15-00 + 10 hours = 27-15-00 minus 24 = 03-15-00 GMT next day on December 11
Step 2 - LMT on naval base Pearl Harbor = GMT minus arc to time of longitude
157°58.5'W arc to time = 157.975 / 15° = 10.531667 hours
03-15-00 GMT on December 11 minus 10.531667 hours = -7.281667 + 24 = 16-43-06 LMT on December 10
Determine the Local Mean Time at the current position of the submarine
Example:
The submarine leaves Pearl Harbor 1941 December 10 at 17-15-00 base time with course 275° and speed 11kts
and is on December 12 at 17-15-00 BT on position 21°41.4'N and 167°24.2'W.
What is the Local Mean Time at this current position?
Difference between longitude naval base and current longitude submarine in arc to time
157°58.5'W minus 167°24.2'W = -9.4283333° x 4 minutes = -37.713333 minutes
17-15-00 = 17.25 x 60 = 1035 - 37.713333 = 997.28667 / 60 = 16.621444 = 16-37-17 LMT
GMT today referred as UTC is the Coordinated Universal Time based on the Greenwich Meridian
and is valid all over the world.
GMT is very important for the celestial navigation and to determine sunrise and sunset.
LMT is the Local Mean Time based on the current longitude.
I don't know if it is implemented in Silent Hunter 4, that would have to be tested.
I hope, I could help a little bit
