H1B/I-94 Visa Validity

Hi all, I am a regular follower for this site for the last one year. Thanks to Redbus, I got many things from this.

My petition approved in Dec’16 after service center transfer and an RFE. My I-797 approved for one year, ofcourse yet to receive my documents (hard copy) from my employer. So, my petition will expire by Dec’17. If my stamping success, I am planning for my trip to US in April’17. What would my Visa and I-94 validity? I mean, when is my clock of one year going to start? Or is it already started? Thanks in advance.