A note on "Ship's time"

It was the usual custom on board ships to keep what was known as Ship time--i.e., the day began at noon BEFORE the civil reckoning, in which the day commences at midnight. Thus, while January 1st, as ordinarily reckoned, is from midnight to midnight, in ship time it began at noon on December 31st and ended at noon January 1st, this period being called January 1st. Hence the peculiarity all through the Journal of the p.m. coming before the a.m. It results that any events recorded as occurring in the p.m. of January 1st in the log, would, if translated into the ordinary system, be given as happening in the p.m. of December 31st; while occurrences in the a.m. of January 1st would be equally in the a.m. of January 1st in both systems.  (Wharton 1893)