I don't know if cargo is different, but at ExpressJet and Continental 8 hrs scheduled, good to start good to finish. If you divert, it is beyond the company's control, and as such you can continue on all remaining originally scheduled legs and exceed 8 hours of flying. They just can't reassign you another turn.
For Example:
EWR Trip: Raleigh Turn & Boston Turn worth 7:58
Holding into EWR, divert to PHL, Continue to EWR after refuel. Still legal for Boston Turn since the diversion was beyond company control. Good to start good to finish. Might fly 9+ hours that day as long as you don't exceed 16 hours of active duty time under the "Whitlow" interpretation of 16 hour duty day.
However, now lets say that you come in from your PHL diversion, and they want to reassign you a PVD turn. This PVD turn is worth less than the BOS turn so logic would say that is ok. Wrong, you're only good for what you were originally scheduled to do. Any "reassignment" (even if logic would say that it is better/less flying) must now include the diversion as part of the "scheduled" flight time in a 24hr period. But if they leave you on the BOS turn, which is more flying, you're good to go. Hence the complete lack of common sense of good to start good to finish.
I don't know if you guys have "contractual rules" but even then, under the Railway Labor Act it is still fly now grieve later, unless it's a FAA rule. The FAA says that diversions for weather (including winds) or MX don't count for "scheduled" flight time, but must be included in "re-scheduled(/re-assigned)" flight time.