sql server dateime issue

In a sql server 2008 r2 database, I have a question about using a datetime field.
I have a column called start_date that is setup in the database as a datetime column.
I want to setup a parameter in a stored procedure that will select the date I pick.
Right now I have the parameter setup like the following:
@Start_Date datetime = NULL.
when I pass a value like 01/17/2012 in the @Start_Date paramter, no records are selected.
However I know there are records where the start_date in the database = 01/17/2012.
It is probably due to the time part of the datetime value.
Thus can you tell me what I can do to pass the value of 01/17/2012 to the stored procedure and/or
tell me what exact value I should pass to the stored procedure so the records I want are selected?

The most simplest thing you could do is declare a variable @end_date , set it to next day

DECLARE @end_date DATETIME
SET @end_date= @start_date+1

and write your where clause as below

WHERE start_date >=@start_date and start_date<@end_Date

The above query will return you all the records for a particular date you pass in @start_date