Is it illegal for your work company to change your time card so you won't hit overtime and double time?

Yes, it is illegal for the employer to change a time record if the change is untrue. Sometimes an employer will change a time record to correct an error by the employee or by the employer. This change is usually to a computerized database and usually there is an automatic audit trail in the computer program. There are several issues as to how to deal with this occurrence. You can tell the employer about it and ask for it to be fixed. This might work or might get you fired. Since you might get fired, it is prudent to make the request, with details to an identified managerial emlployee, in writing and keep a copy. Doing it in writing might get you fired. You have to decide what is best for you. In my experience, employees who put up with unlawful practices by employers can expect future unlawful practices. If you get fired you have a claim for the unpaid wages under the California Labor Code. You also have a claim for retaliation if you were fired as result of your complaint. The same holds true for adverse employment actions short of termination (demotion, cut in hours.) If you are going to act "in concert" with any co-workers about this, I would strongly advise getting legal advice before proceeding. Acting "in concert" has various consequences which should be evaluated for you by legal counsel. The employer's change to time records, if to reduce your actual time worked, not only attacks your rights, but also the rights of other law-abiding employers who pay their employees according to the Labor Code. Therefore, you may also have a claim under Business & Professions Code sections 17200, et seq. I would strongly advise you to seek legal counsel. Any answer to this question is general discussion, and not legal advice. I would need to be retained and to know more facts before giving a legal opinion.