The Z means “Zulu time”, which is the military/aviation/radio term for UTC (Coordinated Universal Time).
So:
1200Z = 12:00 UTC
The letter Z comes from the time-zone designator for zero offset from UTC (“zero” → Z → spoken as “Zulu”).
For you in the UK:
- During GMT (winter): 1200Z = 12:00 UK time
- During BST (summer): 1200Z = 13:00 UK time
So today, while the UK is on BST, 2000Z would mean 21:00 (9 pm) UK time.
You’ll see Z particularly often in astronomy, amateur radio, aviation and meteorology. For observations, writing something like 2026-09-18 2200Z is useful because everyone worldwide knows exactly which instant you mean.