<script type="text/javascript">
        function ShowModalPopup(target)
        {
            $find(target).show(); 
             
             return false;
        }
        
         function CheckDatesMinLessMax(source, arg)
        {
            var fromdate = document.getElementById('<%=frmTxtEducationSchoolDegreeDateFrom.ClientID %>');
            var todate = document.getElementById('<%=frmTxtEducationSchoolDegreeDateTo.ClientID %>');
           
             if(fromdate.value <= todate.value)
		arg.IsValid = true;
	      else
		arg.IsValid = false;
        }
        
        
    </script>
    <asp:CustomValidator ID="cvEducationDates" runat="server" ClientValidationFunction="CheckDatesMinLessMax" ValidationGroup="addEducation" Display="Dynamic"><div class="validationError"><span></span>Date From needs to be before Date To.</div></asp:CustomValidator>    

Add the javascript to the top of the page and add the custom validator where you want to display the error message. I put it just below the field container div