Hi Frank,
Try running an inventory on the drive and then go into the devices section, click on the drive and look in the right pane for the Media Properties. The backup date should be the Allocation Date. I am not 100% sure on where it retrieves this date from (it could be the media database, in which case it may not work) but if it is a timestamp on the tape itself, it should display fine.
Sorry I couldn't give you a 100% answer but hopefully it will help and be correct in answering your question.