Input string format in LINQ query

i try linq query i add one drop-down and two calendar from date and to date like this

<asp:DropDownList ID="regiondrop" runat="server" AutoPostBack="True" 
onselectedindexchanged="regiondrop_SelectedIndexChanged">
</asp:DropDownList>  
<asp:Label ID="Label1" runat="server" Text="From Date"></asp:Label>
<input  ID="fromdate" runat="server" ClientIDMode="Static" />
<asp:Label ID="Label2" runat="server" Text="To Date"></asp:Label>
<input  ID="todate" runat="server" ClientIDMode="Static" />
<input type="button" ID="search_data" runat="server" class="sear_btn"   
 value="Search Data" OnServerClick="search_data_Click" />
<br /><br />

Query

   T1 ts = new T1();
   var list1 = ts.table2.ToList();
   var list1 = ts.table2.ToList();
   var dq = (from reg_data in list1
   join re in ts.table1 on reg_Data.RID equals re.RID
   where reg_data.RID == Convert.ToInt32(regiondrop.SelectedValue)
   || (re.startdate == fromdate.Value && re.enddate == todate.Value)
   orderby reg_data.OName
   select new
    {
    ID = reg_data.ID,
    OName = reg_data.OName,
    RegNo = reg_data.RegNo,
    RID = reg_data.RID
    }).ToList();
    GridView1.DataSource = dq;
    GridView1.DataBind();

when i select value from dropdown and select dates from calendar and click on search button then error occur

Input string was not in a correct format.

in this line

reg_data.RegionID == Convert.ToInt32(regiondrop.SelectedValue) 
|| (reg.StartDate==Convert.ToDateTime(fromdate.Value) && reg.EndDate==Convert.ToDateTime(todate.Value))

any solution?