Original Message

Here is the Notice of Proposed Interpretation to assess the duty day impact on a leg by leg basis, like they do in Part 121:

https://www.federalregister.gov/documents/2010/12/23/2010-32234/interpretation-of-rest-requirements

And, here is where they withdrew it:

https://www.federalregister.gov/documents/2013/11/07/2013-26485/interpretation-of-rest-requirements

The current interpretation is that if your origional planning is reasonable for the entire assignment, and that assignment is delayed for circumstances beyond the control of the certificate holder, the assignment my continue beyond the FAR 135.267 duty day restrictions.   IOWs, If you are good to start the assignment, you are good to finish the assgnment.   

It was true in the Part 121 world also, until the Whitlow letter discussed in the Notice of Proposed Interpretation here, but it was decided, for Part 135 operations, not to use the same standard.