I believe the usage stems from (US'n) television broadcast times; e.g., 10/9c means the show airs at 10 Eastern, 9 central, and (by implication) 10 Pacific. in other words, broadcast is delayed by three hours in the Pacific time zone*. for "live" events on the left coast, such as sporting events, you may be left to do your own math.

*I seem to recall that this became a real issue for the broadcast of shows with more than usual nationwide "import", such as the last episodes of Dallas and The Fugitive.

-joe (dilatorily) friday