Short answer: 12Z and 18Z both hours (times) of the day expressed on a 24-hour
clock, on local standard time in Greenwich, England. (The longitude in Greenwich is 0°; the "Z" attached to the hour refers to "zero" degrees longitude.)
12Z is 1200 hours, or 12 p.m. (noon), local standard time in Greenwich, while 18Z is 1800 hours, or 6 p.m., local standard time in Greenwich.
Pacific Standard Time (PST) is 8 hours behind standard time in Greenwich, so 18Z would be 10 a.m. PST and 12Z would be 4 a.m. PST. When we go on daylight savings time, we move our clocks forward an hour, so Pacific Daylight Time (PDT) is 7 hours behind standard time in Greenwich. Hence, 18Z would be 11 a.m. PDT and 12Z would be 5 a.m. PDT.
This reference time was formerly called Greenwich Mean Time (GMT) but is now called Universal Time Coordinated (UTC) (translated literally from the French expression). The longitude of Greenwich, England is zero degrees–sometimes referred to as "Zulu"–and so the hour appearing on most satellite images often has a "Z" appended to it to identify it as UTC (for example, "12Z", or "00Z", or "1630Z"). The hours, sometimes including the minutes, are given on a 24-hour clock.
For example, Pacific Standard Time (PST), which people on the West Coast use from early November to early March, is eight hours behind Greenwich, so:
|PST = UTC - 08||or||PST = UTC - 0800|
|(if UTC includes only hours, no minutes)||(if UTC includes both hours and minutes)|
(For example, if UTC is 15Z, then PST = 15 - 08 = 07, or 0700, or 7:00 a.m. PST.)
From early March to early November, the West Coast shifts to daylight savings time by moving the clock forward an hour, so Pacific Daylight Time (PDT) trails UTC by only seven hours:
|PST = UTC - 07||or||PST = UTC - 0700|
For example, at 03Z (3:00 a.m. in Greenwich England), the local time on the West Coast of the U.S. is seven or eight hours behind (that is, 7 or 8 hours earlier, depending on the time of year). That would make the local time on the West Coast either 8:00 p.m. PDT or 7:00 p.m. PST–the previous evening! In such cases, the date has to be adjusted as well as the time of day.