In California the law requires an employer to reimburse the employee for the expenses the employee incurs doing the job. However, generally commuting between your home and work is not a business expense for which the employer needs to reimburse you. So your trips from home to the company office would not be something that the employer must reimburse you for. The extent to which the employer must reimburse you for trips form home to a temporary worksite is something that you may want to ask the California Labor and Workforce Development Agency or a labor law attorney.
Once you know exactly what the law requires the employer to pay for you need to raise the issue with your employer. You won't get it unless you ask for it. And since you have the right to reimbursement under the law, the employer is obligated to pay for regardless of whether your boss gets bent out of shape over it.

