Is there an attribute that is the opposite of expiration?

I'm trying to setup accounts to have a specific login time range.

For example;
Start-Time >= 5 June 2011 00:00:00
Expiration == 5 June 2011 02:00:00

I've been hunting googling for hours but I've been unable to find an attribute that would let me specific a "start-time" or a "valid-after" attribute.

Any suggestions?