calculating date fields for 36 months

OK. In that case, assuming the fields are named NextMonth1-NextMonth36, you can use this code as the custom Validation script of the original date field:

if (event.value) {
	var d = util.scand("mm/dd/yyyy", event.value);
	if (d.getDate()<=28) {
		// data is before (or equal to) the 28th -> populate the other fields
		for (var i=1; i<=36; i++) {
			d.setMonth(d.getMonth()+1);
			this.getField("NextMonth"+i).value = util.printd("mm/dd/yyyy", d);
		}
	} else {
		// date is after the 28th -> clear the other fields
		for (var i=1; i<=36; i++) {
			this.getField("NextMonth"+i).value = "";
		}
	}
} else {
	// field is empty -> clear the other fields
	for (var i=1; i<=36; i++) {
		this.getField("NextMonth"+i).value = "";
	}
}