Medical Flex-Spengin Account Question

I posted this earlier with the wrong information. I am eligible for a FSA with work so I am reposting in search of the answer.

Recently had an unexpected emergent surgery resulting in some unplanned medical expenses.

I am eligible to enroll in a FSA begining with my new job (didn't have one with the old one).

If I open an FSA now, can it be used to pay health care bills/copays ect that were billed in May/June 2012 but not yet paid?