date field updation based on dropdownlist changes in classic asp

Hi Team,

I have a dropdown and a text box.

Based on the change in the dropdown 'within_year' selected,the text box 'followup_date' value should be updated.

<tr>
<td><b>Timeline</b></td>
<td><select id="within_year"   >
<% do while not otimeline.eof %>
<option value="<%= otimeline("value")%>" <% if otimeline("value") = ors("within_year") then response.write " selected"%>><%= otimeline("label_en") %></option>
<% otimeline.movenext
loop %>
</select>
</td>
</tr>
<tr>
<td><b>Followup Date</b></td>
<td><input type="text" id="followup_date" value="" o_type="oID:<%= oID%>"/></td><!--value="<%= ors("date_identified")%>"-->
</tr>

current values in the dropdown list are 

1.in the next 7-10 Days

2.in the next 10-30 Days

for example if the first one is selected.followupdate should be updated to todays date+ 10 days = 09/16/2016

or if 2nd option is selected,followupdate should be updated to todays date+ 30days = 10/06/2016

Please suggest the code.Please add the changes to the existing code mentioned above.

Thanks,

Navatha.