how to compare value enter with null value in database

Hi all,

I am developing web App using C#. and sqlserver 2005

My problem is,Assume user enter his data in to database,then it compare the datetime of Now with datetime present in database.(i have a column Createdon(datetime)).and it restrict the user to enter same data in same date.
but problem is Assume if user entering his data for first time how its will check with database time?