18z is 1 PM EST (or 2 PM EDT)
In the military: 18 “zulu”.
18z = 18 UTC, “Coordinated Universal Time” ( what used to be called 18 Greenwich Mean Time or 18 GMT.
Many major models are run at 00z, 06z, 12z, and 18z and their model run names include the zulu time and date of the run.