I don't know much about it but from what you say your server time is constant so you only need to look out for variations in Californian time. What about writing a script as a cron job to check on Californian time (eg a daily visit to worldtimeserver.com set to return a value when the difference is constant, and another value when the difference differs by one hour? The different value can be taken account of by your daily reports and all should be well!

Of course a totally unperlish way of doing this would be to rent (or use free) server space in California and set up regular communications between servers to gauge the difference!