We are not going to be able to tell you when your pay period begins and ends. We have no access to any corporate records.
If you were overpaid, then you can be asked to return the money which was paid to you beyond your earnings. Whether or not that's reasonable or supportable is not something we can assess. What your employer is describing would be very odd, so I would expect a court to be skeptical of the claim that all paychecks were effectively advances on earnings rather than payment for work performed; also, the manner in which the payroll was maintained on the books and the timing of the payment of payroll taxes may suggest something different.

