is there normal way to create a user that can not log on until a future date?

[Rob Fowler]

I am setting up a hotspot in a cafe that is not very high tech.
They offer free wifi in a well known cafe down here in Darlinghurst 
Sydney (All radius users welcome :) )

To prevent abuse I was planning on making daily access keys.
The idea was to make a year of them and print them in a book.
The first person in would simply look up the date in the secret book
put it into the cash register to print on the receipt.

Setting the 'Expiration' time is simple (and works well) but I want to 
prevent someone
from simply copying a key far in the future and using it until then so I 
want to make each
key only valid for a day.
(Yes I know this does not prevent them from photocopying the whole book,
but that's a lot harder than getting a key for this time next year)
Ideally everyone for that day will get the same key
(I made an application to populate the database with simple two word 
tokens).

I think I can probably do it with the SQL interface by only selecting 
from 'check
records' that are current but is there a 'proper' way to do this?
Surely a dialup ISP would set a 'start date', before which a user can 
not log in?

Sorry if this has already been answered but I have searched
the list and don't seem to be able to find a term that will give me an 
answer.

- Rob Fowler - Sydney