End Date Greater Than Start Date?

Work on Asp.Net vs 08. Below is my code

<asp:TextBox ID="txtSTART_DATE" runat="server"></asp:TextBox>
<cc1:PopupControlExtender ID="txtSTART_DATE_PopupControlExtender" runat="server" CommitProperty="value"
CommitScript="e.value += '';" PopupControlID="pnlSTART_DATE"
Position="Bottom" TargetControlID="txtSTART_DATE">
</cc1:PopupControlExtender>
<asp:TextBox ID="txtEND_DATE" runat="server"
ontextchanged="txtEND_DATE_TextChanged"></asp:TextBox>
<cc1:PopupControlExtender ID="txtEND_DATE_PopupControlExtender" runat="server" CommitProperty="value"
CommitScript="e.value += '';" PopupControlID="pnlEND_DATE"
Position="Bottom" TargetControlID="txtEND_DATE">
</cc1:PopupControlExtender>

C# syntax is

protected void dtpSTART_DATE_SelectionChanged(object sender, EventArgs e)
{ AjaxControlToolkit.PopupControlExtender.GetProxyForCurrentPopup(this.Page).Commit(dtpSTART_DATE.SelectedDate.ToString("dd MMM yyyy"));//Set the value
}
protected void dtpEND_DATE_SelectionChanged(object sender, EventArgs e)
{
AjaxControlToolkit.PopupControlExtender.GetProxyForCurrentPopup(this.Page).Commit(dtpEND_DATE.SelectedDate.ToString("dd MMM yyyy"));//Set the value
}

want to compare start date with the end date.Verify that end date greater than start date and today date.How to check the value?