“Zulu” is the one letter code for UTC from the military time zone list. These are standard for the military in Australia, Canada, New Zealand, the United Kingdom and the United States, and commonly used in other armed forces.
The letter time zones also show up in the ISO 8601 date/time format. Just this afternoon, I was writing some search indexing code that used “Z” for the UTC time zone. We use that standard for archive file names so they sort in time order and are unambigious.