I don't know about the big 6 carriers here in the States but we have just a few duty restrictions in our hours of service section of our contract.
We base max duty (report to release) on when you check in for your trip. If you check in (report for duty) at 10pm PHX time then you can only work 10 duty hours. If you check earlier you can work from 12 to 13.5 hours, again based on check in. We limit the back side of the clock duty because we have so much of it. We have an extensive night operation and we have worked hard to limit the company's ability to totally screw with our body clocks.
We do have an extension to 14 hours for wx delays or mx delays but you can refuse due to fatigue. We also do not allow a crew to make another takeoff after they have been up all night flying (we do have 24 hour flip flops, though).
We fly the max hours allowed by the FARs but in practical terms the only limit you ever really hit is the 30 hours in 7 days. I am always interested in hearing how limits are applied outside of the States. It seems that we have more restrictions here which is not what I would have expected, given the strong unions in Europe.