There is nothing you CAN do, I think.
What I do is use the calendar (i.e. current date) and move / delete separately - keep everything local that NxCore expects to be there (14 days historical), move the rest off into an archive location. This means basically copy next day, delete 14 days later.
Validate in the movement process with a PowerShell command that just runs the tape without doing any real processing.
There is not a lo else you can do. Validation has no API, neither has historical download.
I am in the process of setting this up together with an SCOM task (System Center Operations Manager).