The following comments are copied from what I PM posted back to Dog Rocket. Forgive my use of the comments twice, but I didn't want to retype them again, and I think it's applicable here...
Be careful assuming that the Part 91 nature of the flight excuses the flight from duty and flight limitations. Only if it's the last flight of the day and you've ended your duty, does it not conflict. Further, your rest doesn't begin until the Part 91 flight has ended...and that flight does count toward the total commercial flying allowed in a 24 hour period. Note that the 24 hour period is looking back. At any time tomorrow, can you look back and see more than 8 hours flying for a single pilot, or ten for a crew of two...counting that Part 91 flight?
For example, I arrive at the airport at 0600 for duty. My duty must end by 2000, which is 14 hours (more on that momentarily). Between 1200 and 20000, I fly 8 hours. I'm now stuck away from my home base, and need to get home. The company cannot assign me to any more duty or flight time, and I cannot accept such an assignment. However, the company may allow me to make the flight home, at my discretion, so I do. I Depart at 2100 and fly two hours home, arriving home at 2300. I take an hour to put the airplane to bed, and start my rest at 0000.
I'm legal to fly again at 1000, because I've had ten hours of rest. So, the company assigns me to duty at 1000, and my first flight is scheduled for 1100. I have a four hour flight to conduct, which will run from 1100 to 1500. Can I do it?
Looking back from 1500, 24 hours, I see that in the previous 24 hours, I will have flown 11 hours. That's five hours between 1500-2000 the previous day, 2 hours getting home, and now four hours of flying in the new day...that's 11 hours in the previous 24 hours and it's not legal. In fact, starting at 1100 this morning, I can can go on duty, but can't fly. I must be able to look back at any given time and find no more than 8 hours flying in the previous 24, to be legal under Part 135.
I mentioned getting back to the 14 hour duty thing. There's no such thing. Nowhere does the regulation restrict us to 14 hours of duty. Rather, the regulation tells us that in the previous 24 hours looking back, we need to be able to find ten hours of rest preceeding our current duty assignment. By math, we can't work more than 14 hours of duty and still make that work, but nowhere does the regulation spell out a 14 our duty day, explicitly.
This is important, because both duty and flight limitations are determined on a "look-back" basis. That is, we need to be able to look back at any given time over the previous 24 hours, up to that moment when we look back, and see that we're in compliance. This requirement only exists when operating under Part 135 (or 121, as the case may be, although that gets more complicated). It doesn't exist when we're operating under Part 91 at the end of the day, because our obligations under Part 135 are over.
This is why, in the example above you can reach the end of the day at 2000 and look back to find ten hours of rest preceeding your day, and 8 hours of flying in the previous 24. Now, when I take my flight home at 2100, I'm not obligated to look back 24 hours because I'm not operating under Part 135. The following day when I go back on duty and start operating under Part 135 again, I'm required to comply with the look-back requirements and consider all I've done in the previous 24 hours. Because that ferry flight home under Part 91 has been in the previous 24 hours, I have to consider it, too.
Now here's where people sometimes get into trouble. You can't apply the Part 91 flight when starting the day. You can't fly a Part 91 resposition leg to start the day, and then say your duty started after that...your duty started when you repositioned the airplane. For duty and flight time limitations, the anything done up until the end of your duty day still counts against your 135 restrictions. You can operate the airplane under Part 91 as far as cloud clearances, flight plans, minimums, etc...but until your duty day ends, any flights (be it 91 or 135) still count against your duty and flight limitations.