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DateTime start = DateTime.ParseExact(txtStart.Text,"HH:mm tt", null);
DateTime end = DateTime.ParseExact(txtEnd.Text, "HH:mm tt", null);

int interval = 30;
List<string> lstTimeIntervals = new List<string>();
for (DateTime i = start; i < end; i =i.AddMinutes(interval))
   lstTimeIntervals.Add(i.ToString("HH:mm tt"));

dropdownList.DataSource = lstTimeIntervals;
dropdownList.DataBind();

protected void Page_Load(object sender, EventArgs e)
 {
   if (!Page.IsPostBack)
    {             
        PopulateTime();
   }

}

public void PopulateTime()
{

    DateTime start = DateTime.Parse("08:00");
    DateTime end = DateTime.Parse("18:00");

    int interval = 30;
    List<string> lstTimeIntervals = new List<string>();
    for (DateTime i = start; i < end; i = i.AddMinutes(interval))
    lstTimeIntervals.Add(i.ToString("HH:mm tt"));
    
//Assign the data source and bind dropdown       
    cb_Time.DataSource = lstTimeIntervals;
    cb_Time.DataBind();            

}