The only way a 30 hour rest is legal is if you know from a specific point in time looking forward that you will have 30 hours off. Generally with SC assignments they aren't assigned with enough advance notice to have 30 hours. Like you could if you are awarded an 1800 SC at 1130 the day prior and acknowledge the assignment within 30 minutes of the award by 1200. In that case you know looking forward you have 30 consecutive hours so that counts. And that only counts if you acknowledge the award in time. If you acknowledge it at 1201 that's only 29:59 of prospective rest and doesn't count.

With a rotation award there is a greater chance you can push back your 30 hour rest since most of those assignments are now done 2 days out, giving you plenty of time to acknowledge the assignemnt 30 or more hours from report. In those cases that counts as a legal 30 hour rest.

Notice the key in all of this is you acknolwedging the award. This is why I recommend people to not acknolwedge anything, even if legally/properly assigned, until report time. If you are awarded a rotation 2 days out but don't acknowledge it until report time then your last 30 hour prospective rest ended at 0000 right on your first day of LC since at 1800 2 days prior (30 hours out) was the last time you knew with certainty that you had the next 30 hours off. At 1801 you don't know if you have 30 hours because you start LC at 0000, so you only have 23:59 of known rest at that point.

So to answer your question - if you never acknowledge anything then your 30/168 rest would always end at 0000 on your first LC day, even if you are not on LC at that point.