how to set user's expire date/time on first login?

[rabbtux rabbtux]

All,
forgive me if this is a nubie question... It seems many have already
done something similar.  I want prepaid radius accounts for 1 day,
that begins the first time they are used.  For example, someone buys
2- 1 day accounts, and uses them next Monday and Friday.

Is there a simple way to do this check on the radius authentication?
Thank you kindly,
marshall