NetBackup 4.5 and 5.x used a value of 2147483647 which is Mon Jan 18 03:14:07 2038 GMT . NetBackup 6.0 shortened this value to be 2145916799 which is Thu Dec 31 23:59:59 2037 GMT . This change was made in order for underlying Sybase date functions to work correctly.
Strangely have lost 1 second on the expiration time, but if these tapes are about by 2037 ( or me for that matter ) i'll eat them myself. Once i've merged the others i'll go and run the nbcc, haven't done a consistency test for many years so probably worth the effort.