When you get your final approval for the loan, it will be after the property appraisal. The lender will know at that time when your closing date will be and your approval and rate lock should be beyond that date. If you need extensions, most lenders will provide for a small fee.
If you are pre-approved, you will use that pre-approval document in support of an offer made. If the document becomes dated, a listing agent or seller may request a more current pre-approval before accepting your offer.