Q:

Is the time pilots are commuting considered rest time?

Pilots operating under FAR Part 121 assigned to domestic routes are required to have a certain amount of "rest" time before beginning a flight.  Further, the maximum  "flight time," (where the pilot is not resting or on duty) is constrained within specific periods such as monthly, yearly, etc.
If a pilot lives in Florida, but is based in and originates flights from California, is the time spent commuting (from Florida to California) to begin a flight considered part of the FAR required "rest" time?

A:

Within the new fatigue regulations in Part 117 and the older Part 121.471, the word commuting is not defined. Deadheading is defined as follows (Part 117):

Deadhead transportation means transportation of a flightcrew member as a passenger or non-operating flightcrew member, by any mode of transportation, as required by a certificate holder, excluding transportation to or from a suitable accommodation. All time spent in deadhead transportation is duty and is not rest. For purposes of determining the maximum flight duty period in Table B of this part, deadhead transportation is not considered a flight 

Time spent deadheading must be included in the flight duty period calculation (Part 117.25):

(g) If a flightcrew member engaged in deadhead transportation exceeds the applicable flight duty period in Table B of this part, the flightcrew member must be given a rest period equal to the length of the deadhead transportation but not less than the required rest in paragraph (e) of this section before beginning a flight duty period.

Deadheading and commuting are not the same. In section 11 of Advisory Circular 117-3, the FAA provides clarification. It defines deadheading as used to reposition flight crewmembers while commuting is considered an "individual initiated function". 
To address the scenario, it would depend on if the time traveling from Florida to California was scheduled deadhead transportation. If so then the flight time is considered part of the duty period. If such a flight was individually initiated, then the time is commuting and is not part of the duty period. 
Despite the potential legality of the situation, the FAA considers fatigue a joint responsibility between the flight crewmember and the certificate holder. This situation would put the flight crewmember at risk for fatigue. Individual airlines may place additional policy restrictions on commuting since Part 117 does not.

