Conversion failed when converting date and/or time

C# code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;
using System.Configuration;

public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{

}
protected void btnsave_Click(object sender, EventArgs e)
{
SqlConnection con1 = new SqlConnection(ConfigurationManager.ConnectionStrings["NewTaskConnectionString"].ConnectionString);
try
{
if (con1.State == ConnectionState.Open)
con1.Close();
con1.Open();
string strQuery = "INSERT INTO Task(Taskname,Taskdescription,Assignedto,Taskdate,Starttime,Endtime,Hours,Priority) VALUES('" + txtName.Text + "','" + txtdescription.Text + "','" + Session["Username"].ToString() + "','" + txtdate.Text + "','" + drpdwnhr.SelectedItem.Text.Trim() + "-" + drpdwnmin.SelectedItem.Text.Trim() + "-" + "','" + drpdwnhr.SelectedItem.Text.Trim() + "-" + drpdwnmin.SelectedItem.Text.Trim() + "','" + txthours.Text + "','" + Priority.SelectedValue + "')";
SqlCommand sqlcmd = new SqlCommand(strQuery, con1);
int i = Convert.ToInt32(sqlcmd.ExecuteNonQuery());
con1.Close();
if (i == 0)
{
Response.Write( "Error");
}

else if (i >= 1)
{
Response.Write ("Data Saved Successfully");

}
}
catch (Exception ex)
{
throw ex;
}
}
protected void Calendarbtn_Click(object sender, EventArgs e)
{
try
{
if (txtdate.Text.Trim() != "")
cdrCalendar.SelectedDate = Convert.ToDateTime(txtdate.Text);
}
catch
{ }
cdrCalendar.Visible = true;
}
protected void cdrCalendar_SelectionChanged(object sender, EventArgs e)
{
txtdate.Text = cdrCalendar.SelectedDate.ToShortDateString();
cdrCalendar.Visible = false; //hiding the calendar.
}
}

Thanks,
Laxman