Is this a employment contract or just an offer letter? There is a hugh difference.
If it is an offer letter, than you can't be held to any type of notice with California being an at will state. The employer or the employee can leave without any notice at all.
If it is a bonifide contract than you do need to adhere to the terms of the contract. But I would guess it is not a contract from your statement that it is an offer eltter.

