Sql Server 2005 Add Current Time To The Passed Date

 Greetings,
I have an e-timesheet application.
i made a function that takes the startdate of the week:
public SqlDataReader GetProjectsActive(DateTime SheetDate)
        {
            SqlConnection myConnection = new SqlConnection(ConfigurationSettings.AppSettings["connectionString"]);
            SqlCommand myCommand = new SqlCommand("GetProjectsActive", myConnection);
            myCommand.CommandType = CommandType.StoredProcedure;

            SqlParameter parameterSheetDate = new SqlParameter("@SheetDate", SqlDbType.DateTime);
            parameterSheetDate.Value = SheetDate;
            myCommand.Parameters.Add(parameterSheetDate);

            myConnection.Open();
            SqlDataReader dr = myCommand.ExecuteReader(CommandBehavior.CloseConnection);
            return dr;
        }
i run the application and i pass the value 6/1/2008 (date without time) to the GetProjectsActive but what i notice that the storedprocedure adds the current time to the passsed date when it has been executed so some projects will not  appear since the startdate and enddate of the project are only dates so i try this solution:
 parameterSheetDate.Value = SheetDate.Date;
and it works fine
but my question why the current time has been added to the storedprocedure automatically although i pass only the date 6/1/2008 date????
your help is highly appreciated.
Best regards.