Generally, a petitioner can request a 7th year extension (plus any additional time remaining in the initial 6 years) of H-1B status based on Labor Certification/I-140 which remains pending for 365 days or more. To do so, a petitioner must establish that the above criteria were or will be met either on or before the requested start date on the H-1B extension application. To answer your question, you don't have to wait until September 12, 2013 to file for H-1B extension.
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Your employer can.
Your employer needs to retain an experienced immigration lawyer to review all the facts, advise them, and handle the case. Your employer can find one through http://www.ailalawyer.com.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
H-1B extension petitions can be filed six months in advance. However your employer will have to time it to file after the 365 days of wait for PERM.
This response is general in nature and cannot be construed as legal advice, given that not enough facts are known. Any comments offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship. If you would like additional information based on this response, please contact my office at 510 657 7665 or 415 902 0832 to schedule a consultation.