but I only received my I-797 on 3/28. Can I start to work on 3/10?
-If the receipt date on the receipt was 3/10, you can work legally from that date.
Since the employer is not quite sure about this, I may propose to say that since we use FedEx overnight service to file the H-1B transfer, the receive day is definited the next day of the FedEx shipment.