As a previous correspondant suggests, Zulu time is GMT or as as it is now
known, UTC - Universal Coordinated Time.
GMT/UTC matches European time during the Winter and is an hour behind in the
Summer. It is of no significance to photos taken in European airspace where
time differences are of the order of less than four hours or so (except
Eastern Soviet territory) but might be more important when it comes to
evaluating overseas photos.
(As a background the world is divided into 24 (?) timezones, each with their
own letter in the phonetic alphabet to demark it. Zulu (oddly not Alpha) is
the line of longitutude through Greenwich.)