Expiration Value

I'm attempting to use Expiration to expire user accounts after a set time period. What format does the Date/Time (Value field) have to be? From what I can see it's in the format of "Monthname Day Year Hour:Min:Sec". So for example "March 24 2008 00:00:00". But it appears that in this format you can't use normal SQL datetime operators to see if it's expired (for example, to run a SQL query to remove all expired accounts). Can someone who has it working please let me know what format they use for Expiration value, and how they can use MySQL comparison operators with it? (Ether 2.0.2 or 1.1.7). Thanks Tim