you can use an asp:CompareValidator.
as like
1.
<asp:comparevalidator id="compvalidDtGft" runat="server" cssclass="red" forecolor=" " controltovalidate="txt2" controltocompare="txt1" xmlns:asp="#unknown">
ErrorMessage="Date selected cannot be greater than first date." Operator="LessThanEqual" Type="Date"
Display="Dynamic"></asp:comparevalidator>
and in addition to this
2.
<asp:comparevalidator id="CompareValidator1" runat="server" controltovalidate="txt2" xmlns:asp="#unknown">
ErrorMessage="CompareValidator" Operator="GreaterThanEqual" Type="Date"></asp:comparevalidator>
And Try to put this on page_load event
protected void Page_Load(object sender, EventArgs e)
{
CompareValidator1.ValueToCompare = DateTime.Now.ToShortDateString();
}
or try
2.
<asp:comparevalidator id="CompareValidator1" runat="server" controltovalidate="txt2" valuetocompare="<%= DateTime.Now.ToShortDateString() %>" xmlns:asp="#unknown">
ErrorMessage="CompareValidator" Operator="GreaterThanEqual" Type="Date"></asp:comparevalidator>
IF all these doesn't work try to write a custom validator