The manitfest.plist tracks encryption settings and applications installed on the device. The datetime stamp included in this file tracks the last time encryption settings changed. If the backup were encrypted and the user decided to create a new unencrypted backup, that would be the date reflected here. This would also be the case if the user decided to encrypt a backup that was previously unencrypted. Forensic tools may also change this file if the tool provides the option to encrypt the backup during the forensic extraction.