Re-keying of Crypto, for commo nets and for things like IFF (Identification Friend or Foe)is generally done at midnight zulu - to keep everyone in sync. Also things like challenges and passwords, etc. are (usually) changed based on Zulu time. I say usually because I have seen more than one instance where a lack of clarity in an order caused some people to use local time and other Zulu time. At the tactical level, at least from a ground perspective, it is easier to just use local time - but important to make that clear or someone will read too much into it and do the conversion. On formal Operations order, time zone used will be stated in the heading, and all later date time group references will have the time immediatley followed by an "L" for local. Whenever you are dealing with larger scale ops though, zulu time is all I have seen used, as time zones are easily crossed.
Example, A few years back when I was playing curator of the Patriot Air Defense System at Al Jaber Air Base, Kuwait, I changed out the key to the IFF interrogator at 0300L, which was 2400Z. Our less secure SIF codes also changed same time. Incidentally, the Air Force liked to forget to change their own codes (as they are the ones who would publish them). I used to get a kick out of interrogating aircraft that I knew were friendlies, receiving a negative response and waiting a few minutes before trying again - and always getting the correct response. Gotta keep those damn Jet Jockeys honest.
BTW, I would say 8:45 PM in casual coversation, 2045 Local if clarity was required for a mission. I have always wondered about people who use 24 hr. time when dealing with "straights" (civilians) - just what are they trying to do - do they think it makes them look cool or something?
And yes, Zulu time = Greenwich Mean Time = Greenwich Meridian Time = Coordinated Universal Time. Any differences are anal bickering, like taking about when the millenium actually started.
Adam