This is a confusing thing, but basically your duty period is 16 hours that begins when your reserve period starts; so 0130, 0730, 1330, or 2030 and add 16 hours to it and your toast. That's why you have to be blocked in minus 30 minutes before the end of 4 hours past your reserve period. (R 12hrs + 3.5 + 30min debrief = 16hrs)

Anybody please shed light on this if I'm wrong.