What is the PED on the I-797B? Your visa do not override the PED in your I-797. Also your visa do not govern your legal status in the US, it is just a travel document required to enter the US. It is the I-94 document that governs your legal status in the US.
So, when you enter the US, you will present the I-797B to the CBP at port of entry along with your passport/visa and other H1B related documents and they will issue I-94 matching the PED on your I-797B. Once in the US, your employer will need to apply for extension of status before your I-94 expires.