A ship’s clocks will not necessarily be altered the moment she enters a new zone. The fact that she is crossing a dividing meridian and entering a new zone does not alter the time she is keeping. Her clocks are altered to suit the convenience of those onboard.
Local mean time.
Local mean time (LMT) is a more precise indication of time and it reflects the actual time on a meridian. To derive LMT in a particular position, it is necessary to establish the GMT from the zone time one is operating in and thence to apply a correction derived from the actual longitude of the position (the longitude is converted to time). For example:
A ship is steaming in zone (-2) and in position 18° 20’ S 21° 35’ E, the ship’s clocks indicate the time as 1215 B. What is the GMT and LMT?
Remember that GMT is 2 hours behind zone -2 and therefore when it is 1215 in zone -2, it is only 1015 at Greenwich. Remember that 15° of longitude represents the angular distance the sun apparently moves in one hour.