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

There is no 14 hour duty day limit under Part 135.  None.

There is a look back requirement for 10 hours of rest in the last 24 hrs at the completion of the planned Part 135 flight (not the duty thereafter).

That's were people get it wrong, thinking there is a 14 hr duty day limit.

You can be on duty forever.   But, to PIC an aircraft under Part 135, you have to show you had a 10 hr rest period looking back 24 hrs.   If not, your rest clock has not reset.   You also have to show at least 13 rest periods of at least 24 consecutive hours each in each calendar quarter.   Not "days off" but 24 hour periods (i.e, free from all duty).

The notion of a 14 hour duty day limit is born from fiction and peddled by the ignorant