why no overtime?

This is my first job as a LVN, so I'm not too familiar w/ the system. I'm also still fairly new at this facility (2 months). I work at a SNF as a part-time/on-call LVN. This was the first time they called me in to do a 12 hr shift, 7p-7a. I check my paycheck a couple weeks later and i see only 30 min of OT. From what i know, anything over 8 hrs is overtime. I spoke to my supervisor about it and he explained to me that our time-clock system is set up so that it looks like i didnt do overtime. There's a cut off at 12:01am. so technically, according to this system, i worked from 7p-12a, then 1230a to 730a...(one 5 hr shift and one 7 hr shift on separate days). I don't understand this because it's still the same damn shift and I hate that im not getting paid overtime because of some time-clock loophole.

I just wanted to know if this has happened to anyone else who works graveyard, and if this is reportable to the dept of labor (in CA); meaning, can I file a wage claim? Or is this totally legal and this is how it works at all facilities?