I have researched in newspaper archives, and found an article in the paper 'The Mountain Democrat Times' of 1968 March 28, which confirms that the whole area on the Yuma AZ side of the Colorado River but belonging to California followed even then Arizona time, i.e. MST without observing DST. It mentions the post office in Bard, CA as an example, and probably it includes the populated places Winterhaven, Ross Corner and Bard. Before 1968 I found no source. But probably we can assume that this habit has been permanent. The simplest is to include this area with Arizona, for timezone purposes. On 07.04.11 06:45, Steve Jones wrote:
CA: Winterhaven California, in deference to its proximity to Yuma AZ, observes Mountain Standard Time year round. This means that (like AZ) Winterhaven does not observe Daylight Saving time. So this puts Winterhaven ahead of California by one hour (in sync with Arizona) during Standard time - in the Fall and Winter. And in sync with both CA and AZ during Spring and Summer (Daylight Saving time). The dividing line of this observed boundary is locally considered to be the "All American Canal".