How to compare a time stamp field in where clause?

Hi,
I am writing one query:
select * from user where logged_in_time='15aug2011 00:00:00' ;
as logged_in_time is a timestamp field and I want to list out all the users who logged in at this time.But it is not displaying any output, whereas if I write:
select * from user;
and try to find it manually, rows are present with this logged_in_time. Please help.
Regards,
Jake.
