You have my sympathy. Alas, there is nothing in a cpio archive to indicate the time the archive was created. About all you can do is make a listing of the archive contents ("cpio -itv") and process the output to find the most recent date therein. Even that is not going to be 100% reliable because it is always possible that someone marked a file with a time stamp far in the future. If these are complete system backups, perhaps you could use the time stamp of the newest file in /var/log as a surrogate for the creation time of the archive.
And, I just realized that only the date and year (no time) are shown for time stamps not within the past year. You might have to write your own program to parse the (simple) cpio archive format if you need complete time stamps for older files.
Last edited by rknichols; 12-28-2011 at 10:25 AM.
Reason: And, I just realized...