This is a rather complicated issue for an open forum. A lawyer would need to have a longer conversation with you about the facts of your particular situation. You should consult an attorney experienced in wage-hour and class action issues.
While all situations may be different, I will not be as reluctant to answer this question as the others who have answered it. I can think of very few, if any situations that would justify an employer altering a time card, under California law. An employer is required to maintain time records for non-exempt employees. Time records should be accurate. If an employee is making errors, this should be corrected by employee counseling or discipline, documenting errors. If an employee is falsifying a time record, any discipline, including fire, is more than reasonable. Otherwise, an employer who alters time cards to its benefit is creating a practice that would subject it to liability for unpaid wages. You are entitled to pay for all hours worked. The only exception to this rule is known as "rounding." An employer may round out minor pluses and minuses. People are more likely to clock in at 8:59 or 9:01 than precisely at 9 a.m. An employer may treat that logically by rounding up or down. However, an employer may not engage in a rounding scheme that always works to its benefit. Rounding is limited to 5 minutes ore 1/10th of an hour. Otherwise, it sounds like you might be owed some wages.