Geologists say California will have a major earthquake in the next 30 years?

They have been reporting this for numerous years already. When did they first say that? And does the time reset each time they have a "study" that predicts a major earthquake in the next 30 years? If they said it in 2005, then shouldn't their August 2010 study say it would happen in the next 25 years? It seems the "30 year" prediction never decreases. Does anyone have any insight on this? Just curious. Without being able to accurately predict earthquakes, I understand they really don't know when it will happen. Is 30 years just a guideline used for stating the probability of a future earthquake?