Please refer to ISBP rule no. 21 b
ISBP Rule 21 b. refers to stale documents acceptable which means that the documents are presented later than 21 calendar days after the date of shipment are acceptable.
You would only need to apply this when original transport documents are required since Article 14c states that documents must be presented within 21 days of shipment if at least 1 original transport document is presented. If no original transport document is required, then the latest presentation date of the documents falls on the expiry date of the credit.
You may also be asking what if the bank states that documents must be presented within x days of shipment without requiring an original transport document. In this case, a clarification must be requested from the issuing bank as this clearly contradicts that stale b/l is acceptable.