Yes, if you work for a Part 135 Certificate holder, you have a duty day limitation TO FLY PART 135. There is no duty day limitation to fly PART 91. It isn't there.
So, you are correct to say what you do, but wrong to imply it limits your day to 14 hours under Part 91 operations!
You can operate aircraft Part 91 as you see fit with regards to duty time limitations! Because, there are none limiting you.
FAA FAR 135 Subpart L Final Rule:
"Additionally, the FAA acknowledges that certificate holders may not be able to conduct certain operations because of the more stringent part 135 requirements. For example, the weather minimums may be below part 135 standards, but would have been acceptable for a part 91 operation. Similarly, additional part 135 flights may mean that a flightcrew member reaches flight time limitations more quickly. Nevertheless, the FAA has determined that these restrictions are appropriate given the increased safety of operations that are expected as a result of this regulation. However, the FAA is not extending this regulation to flights conducted without medical personnel onboard. The FAA has determined that such an extension would go beyond the stated rationale of providing additional protections to the medical personnel and passengers onboard the helicopter."