You could drop orders 5 minutes after you got your ID on day #1 if you really wanted to. The Federal Law is called USERRA. Go to the Employer Support of the Guard and Reserve website
www.esgr.com. The law basically guarantees your right to serve in the military anytime you want to and your employer has to hold your job for you. The great deal is that they have to treat you like you were never gone. So if you went on 10 months of mil leave 2 months into the job and returned on your anniversary date, then you would be on 2d year pay. They would probably consider you still on probation though. Why suffer through year 1 pay at CAL or UPS if full time orders are an option?
I would be doubtful that CAL fired someone for doing it. The Dept of Labor has been aggressive about enforcing the law. I don't know if I would even really involve the Union as much as I would be contacting my congressmen and the Dept of Labor.