which datatype should be used to save time only in SQL server

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SQL-Server-2005

C#

ASP.NET

, +

I have created a database to save details of online-exam generation.
Here i want to save duration of exam time and i am using datetime datatype for this, but it takes current date also which is not needed.
...below is my table structure...

create table Exams
(
slNo int primary key identity(1,1),
examID varchar(10) unique  not null,
title varchar(500) not null,
examHours datetime not null,
totalQn integer not null,
totalMarks integer not null
);

The user inputs the time in three textboxes in format txtHH , txtmm , and txtSS respectively.
here is codebehind page...

SqlConnection con = GlobalConnect.FnConnection();
            con.Open();
            string exmTime = string.Format(txtHH.Text.ToString() + ":" + txtMM.Text.ToString() + ":" + txtSS.Text.ToString());
            SqlCommand cmd = new SqlCommand("Insert into dbo.Exams(examID,title,examHours,totalQn,totalMarks) values(@examID,@title,@examHours,@totalQn,@totalMarks)");            
            cmd.Parameters.AddWithValue("@examID", lblExmId.Text);
            cmd.Parameters.AddWithValue("@title", txtTitle.Text.Trim());
            cmd.Parameters.AddWithValue("@examHours",Convert.ToDateTime( exmTime));
            cmd.Parameters.AddWithValue("@totalQn", TxtNoOfQn.Text.Trim());
            cmd.Parameters.AddWithValue("@totalMarks", TxtTotMarks.Text.Trim());          

            int effect = GlobalConnect.ExecuteNonQuery(cmd);

Please suggest me how to do this.