my paychecks are supposed to be biwekely, but they seem to be dating from over a month ago, is this legal?

Your employer is required to pay you in a timely fashion. The law says payments made beyond 15 days from the regularly scheduled payday are late and you would be entitled to interest and penalties.  If you've not discussed the timing issue with your employer, you should start there.  If you can't get the matter resolved, you can contact the PA Department of Labor, wage and hour division to file a complaint.