add x days to date from input field

HTML

<label>Start Date</label>
<input type="text" maxlength="50" readonly="readonly" size="27" class="inputbox text" value="2018-11-30" name="start_date" id="start_date"><br>
<label>Start Date +90 days</label>
    <input type="text" maxlength="50" readonly="readonly" size="27" class="inputbox text" value="" name="future_date" id="future_date">

JavaScript

startDate = $('#start_date').val().split('-');
days_to_add = 90;
$myyear = startDate[0];
$mymonth = startDate[1];
$myday = startDate[2];
date = new Date();

date.setFullYear($myyear,$mymonth,$myday);
date.setDate(date.getDate()+(days_to_add));
console.log(date);
$('#future_date').val(date.getFullYear()+'-'+date.getMonth()+'-'+(date.getDate()+1));