try this
<div class="row form-group">
<div class="col-sm-2">
<label for="smallinput" class="col-sm-2 control-label label-input-sm">Date</label></div>
<div class="col-sm-4">
<label for="selector1" class="col-sm-2 control-label">Zone</label>
<asp:DropDownList Enabled="false" ID="DropDownList2" runat="server" CssClass="form-control1"></asp:DropDownList>
</div>
<div class='input-group date col-sm-4' id='datetimepicker8'>
<asp:TextBox ID="txtDOB" runat="server" CssClass="form-control"></asp:TextBox>
<asp:RequiredFieldValidator ID="RequiredFieldValidator13" Display="None" runat="server" ErrorMessage="DOB is required." ControlToValidate="txtDOB" ValidationGroup="Save"></asp:RequiredFieldValidator>
<span class="input-group-addon">
<span class="fa fa-calendar"></span>
</span>
</div>
</div>