My landlord called today to say she excepted an offer on the house we are currently renting. She said she thought the closing date is Sept. 14. How much time do we have to find a new place?

If you have a lease, the new owner will have to honor the lease.
IF you are on a month to month payment, they will have to give you a 30 day notice. Did your landlord give you notice to move ? If not the new owners will still have to honor the 30 day notice