Date Validation

Hi

I have 2 Dates . Added On & Valid Upto . I want Added on as default system Date & Valid UpTo should be greater than Default System Date

<div class="col-lg-3 col-md-6 col-sm-12">
    <div class="form-group">
        <label>Added On</label>&nbsp;<span style="color:red">*</span>
            <asp:TextBox ID="txtAddedOn" class="form-control daterange-single" required="true" onkeydown="return false" placeholder="Added On" runat="server"></asp:TextBox>
    </div>
</div>

<div class="col-lg-3 col-md-6 col-sm-12">
    <div class="form-group">
        <label>Valid UpTo</label>
        <asp:TextBox ID="txtValidUpTo" class="form-control daterange-single" onkeydown="return false" placeholder="Valid UpTo" runat="server"></asp:TextBox>
    </div>
</div>

Thanks