Hello,

Unfortunately there isn't an easy way to accomplish this. Here's the closest to an easiest way:
1. Create a field on your form and mark it as 'validate-date with calendar popup'.
2. Go to http://app.formassembly.com/forms/prefill_info/FORM_ID
(where FORM_ID is the number of your form)
3. Record the id for the field you just created.
4. Paste this code into your custom code block (replacing tfa_FIELDID with the value from step 3, and DAYS with number of days in the future to limit by):
<script>
var elem = 'tfa_FIELDID';
//Do not change below this line
base2.DOM.Element.addEventListener(document,'DOMContentLoaded',function(){
wFORMS.behaviors.validation.rules.isDateInTheFuture = { selector: "#"+elem, check: 'validateDateInTheFuture' }
wFORMS.behaviors.validation.messages.isDateInTheFuture = "This date cannot more than DAYS in the future.";
wFORMS.behaviors.validation.instance.prototype.validateDateInTheFuture = function(element, value) {
var testDate = new Date(value);
var today = new Date();
today.setDate(today.getDate()+DAYS)
return (testDate.getTime() < today.getTime());
};
});
</script>

Ah, ok, then this is probably a better solution:

Limit date selection to certain number of days after current:

<script>
function setFuture(days){
var a = new Date();
a.setDate(a.getDate()+days);
// Calendar
if(!wFORMS.helpers.calendar) {
wFORMS.helpers.calendar = {};
}
if(!wFORMS.helpers.calendar.locale) {
wFORMS.helpers.calendar.locale = {};
}
var cfg = wFORMS.helpers.calendar.locale;
cfg.mindate= a;
}
setFuture(15);
</script>

Which should force the calendar to not event let users select a date less than 15 days in the future.