The Time in Gibraltar is
CET or CEST (summer)
Timekeeping in the USA

Timekeeping in Britain

Its a matter of time ...

You may notice that its reported in the text that the 'Mary Celeste' was sighted by the 'Dei Gratia' on one date in the text, and another on the chart. the reality is it was first seen on both the 4th and 5th December;

This anomaly is explained by the fact that their record keeping was based on "Civil Time", presumably EST of the day and also "Sea Time", presumably the celestial time of the ships current location.

It would of course have been easier to express times in GMT, however that was not adopted as a world standard until 1884.

GMT is based on mean solar time (at Greenwich, UK) and has since been replaced by UTC based on quantum resonance and a consensus of clock keepers. Click on the links for more information.