Letade ett bra tag innan jag postade.
Typiskt att jag ska hitta en lösning direkt efter :)
<form>
<input type="text" name="date1" size="20"><br>
date (yyyy/mm/dd)<br><br>
<input type="text" name="date2" size="20" readonly><br>
90 days from date<br><br>
<input type="button" value="Calculate" onclick="plus90(this);">
</form>
<script type="text/javascript">
document.forms(0).date1.value = dateToString(new Date())
function dateToString(d){
return [d.getYear(),d.getMonth()+1,d.getDate()].join('/')
}
function plus90(x){
d = new Date(x.form.date1.value)
d = new Date(d.getYear(),d.getMonth(),d.getDate()-2)
x.form.date2.value = dateToString(d)
}
</script>