Error: String was not recognized as a valid DateTime in Gridview Update Event in ASP.Net

hello Everyone,

I’m using datetime picker. For reference https://xdsoft.net/jqplugins/datetimepicker/

When I’m updating the row its gives String was not recognized as a valid DateTime. For reference I’m adding the code

protected void UpdateCustomer(object sender, GridViewUpdateEventArgs e)
    {
        SqlConnection con = new SqlConnection(ConString);
        int userid = Convert.ToInt32(GridView1.DataKeys[e.RowIndex].Value.ToString());
        GridViewRow row = (GridViewRow)GridView1.Rows[e.RowIndex];
        Label lblID = (Label)row.FindControl("lblID");
        //TextBox txtname=(TextBox)gr.cell[].control[];  (DropDownList)row.FindControl("DropDownList1");
      
        //TextBox txt_Objective = (TextBox)row.Cells[2].Controls[0];
        //TextBox txt_Responsible = (TextBox)row.Cells[3].Controls[0];
        TextBox txt_title = (TextBox)row.FindControl("TextBox1");

        TextBox txt_meetingstart = (TextBox)row.FindControl("txtMeetingStartTime");
        string date = (DateTime.ParseExact(txt_meetingstart.Text, "MM/dd/yyyy HH:mm:ss.fff", null)).ToString("yyyy-MM-dd HH:mm:ss.fff");
        string dob = (DateTime.ParseExact(date, "y-m-d H:i", null)).ToString("yyyy-MM-dd HH:mm:ss.fff");
        TextBox txt_meetingend = (TextBox)row.FindControl("txtMeetingEndTime");
        string room = (GridView1.Rows[e.RowIndex].FindControl("ddldepartment") as DropDownList).SelectedItem.Text;
        //TextBox textc = (TextBox)row.FindControl("txtc");  
        GridView1.EditIndex = -1;
        con.Open();
        //SqlCommand cmd = new SqlCommand("SELECT * FROM detail", conn);  
        SqlCommand cmd = new SqlCommand("update Meetings set MeetingTitle='" + txt_title.Text + "',MeetingStartTime='" + txt_meetingstart + "',MeetingEndTime='" + txt_meetingend.Text + "',Room='" + room + "' where PK_MeetingID='" + userid.ToString() + "'", con);
        //cmd.ExecuteNonQuery();
        con.Close();
        BindData();
    }

 <script type="text/javascript" src="calendarEdit/jquery.js"></script>
    <script type="text/javascript" src="calendarEdit/jquery.datetimepicker.js"></script>
    <script type="text/javascript">
        $(document).ready(function () {

            $("[id$=txtMeetingStartTime]").datetimepicker({
                format: 'Y-m-d H:i',
                
                datepicker: true, 
                showOn: 'button',
                buttonImageOnly: true,
                buttonImage: 'css/images/Calendar-icon.png'
            });
            $("[id$=txtMeetingEndTime]").datetimepicker({
                format: 'd/m/Y H:i',
                datepicker: true,
                showOn: 'button',
                buttonImageOnly: true,
                buttonImage: 'css/images/Calendar-icon.png'
            });

        });
    </script>   

Thanks.