My h1 and I94 were expired on sept 30th 2015 and my extension is in process with current employer… I got a new offer and my new employer filed h1 transfer… Will my h1 transfer gets approved from sept 30th or from any other date… Also can I join new employer after they file h1 transfer or wait till I receive receipt notice…
H-1 transfer will have approval date of the actual date on which it will be approved. It will not be backdated to Oct 1 or Sep 30.
H-1 extension through old employer will have an effective date of Oct 1 to maintain continuity of status and employment.
In general, you can join new employer as soon as the petition is received by USCIS. However, most of the folks prefer to wait to have the receipt notice before joining the new employer.
Your case has additional complexity due to expired I-94, which I have previously answered. You should consider those in mind when deciding when to join the new employer.