Can LCA be filed before joining a new client?

[dokter]

Hi
My current H1B expires by Sep'2013, but for personal reasons i need to travel to india
by august, so i'm planning to do my extension in july'2013 in premium and get my visa stamped in
mexico/canada as i don't want to do my stamping in India.

between
I'm changing my client from A to B, with the same employer 'C' and i want to apply for an extension with the same employer 'C', but i haven't joined client B yet, as i haven't received the start date, but whether LCA can be filed before i actually join client 'B'?

Because time is running out for me, whether i have sufficient time left to get my visa approved
and stamped before the july 31st

thanks
dok