how to add days in

can you please help me with this. I have this sets of inputs here;

Days:<select id="select">
         <option value="150 days">150 days</option>
         <option value="200 days">200 days</option>
       </select>
Date1:<input type="date" id="date1">
Date2:<input type="date" id="date2">

I dont know how to properly explain this, but what's supposed to happen is this:

if user selects 150 days, it will be added to date1(which is also inputted by the user) and will be automatically shown in date2.

note that date1 is not the current date...