Advance date 90 days when date change in Form

Use cfinput's bind attribute, for example

<script type="text/javascript">

/* add 90 days to a given date */
addDays = function(dateStr) {
var newdate = (new Date(dateStr)).add('d',90);
var days = newdate.getDate();
var months = newdate.getMonth()+1;
var year = newdate.getFullYear();

/* the datefield widget requires 2 digits to display*/
if(days < 10) days = '0'+days;
if(months < 10) months = '0'+months;
return months + "/" + days + "/" + year;
}
</script>

<cfform name="dateForm" id="dateForm">
<cfinput type="datefield" name="startdate" value="#dateformat(Now(), 'mm/dd/yyyy')#">
<cfinput type="datefield" name="nextdate" bind="javascript:addDays({dateForm:startdate})" readonly="yes"> 
</cfform>