Querying now()

Hello,

I have a problem I feel Im going about the wrong way but after reading
over the date/time data types documentation a few times Im still unsure
of how to do this.

Id like my application to do something like this :

SELECT now() AS currentTime WHERE currentTime < '$timestamp';

So if it returns a row Id know $timestamp is not in the past.  However
when you run this it simply says 'ERROR:  column "currentTime" does not
exist'.

Could someone offer some insight as to the correct way to accomplish this?

Thank you.
A Gilmore