The Z flag is one of the international maritime signal flags.
In the system of international maritime signal flags, part of the International Code of Signals, the Z flag stands for the letter Z ("Zulu" in the NATO Alphabet) when used in letter-by-letter alphabetic communication. When used alone, it means "I require a tug" or, when used by fishing vessels near fishing grounds, "I am shooting nets".
The Z flag when combined with four number flags (The leading two denoting hours, the trailing two denoting minutes) indicates Z Time (also called Zulu Time), a military and maritime term for Coordinated Universal Time (UTC) (formerly called Greenwich Mean Time (GMT)). Thus this would mean 0800Z ("zero eight hundred zulu") equivalent to 08:00 UTC:
Or, more likely, the same information would be conveyed using repeat flags: