H1 expiry and renewal

#1
Hi ,

My visa was stamped on August 2015 but I still not moved to US, due to delay by my employer. My Visa has two expiration dates. One expiry date which is August 2020 and 2nd is PED which is August 2017. Can someone please tell me which one is right expiry date, or what is difference in both of them.
Secondly now my employer is asking me to come and if Aug 2017 is expiry then I have only two months for renewal, I have heard that we need client letter for extension. Please tell me if it is really required for extension, as two months are too short for me to get a role.
Kindly suggest.

Thanks.
 
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