Attention: There Is No 14/hr Duty Day Limit!!  

A certificate holder is operating under 135.267 [d]; not 135.267 [c].  135.267 [c] actually specifies "a regularly assigned duty period of no more than 14 hours."   135.267 [d] applies to a flight crew member who, as an example, is on call or similar and is not on a regularly assigned duty schedule. Those two cannot be conflated into one revolving or interchangeable set of scheduling methods simultaneously by the operator.  Only one can apply for the operator.  Read them closely.

Additionally, a lot of individuals seem to believe duty time may be extended.  Again, please read the entire 135.267 section.  Only "Flight Time" may be extended explicitly by this section.  There is no reference to duty time extensions under Part 135.267.  That's the reason some operators [notwithstanding 135 Subpart L] allow the legs following passenger drop-off to be conducted under Part 91.  The Part 135 requirements stop when the paying passenger is dropped off [135.85].

As cynical as this sounds, there will most likely be a lot of down-votes.  The regulation is specific and is contexed explicitly.  Not to be insulting or condescending, but the down-voters may want to do further research for clarification.  

V/r