Immigrant Extentions

Permission to enter and/or remain in the United States and extensions of stay in this country are granted by the Department of Justice, Immigration and Naturalization Service (INS). Upon entering the United States, the INS Office at the port of entry places in your passport a small white card, Form I-94. On this card the INS officer writes in either a date or "D/S" (duration of status). The date on the Form I-94 is controlling. That is, if you have "duration of status" you may remain in the U.S. as long as you are in the same job or same category of job. If you have a date on your Form-94, you should apply for your extension of stay with INS prior to its expiry.

The Department of State has no authority to extend your stay or change the information/date on the Form I-94.
A visa is used solely to make application to INS to re-enter the U.S. after being outside of this country. That is, a visa is used only to cross borders and has nothing to do with the length of your stay in the U.S.