You are trying to parse an epoch millisecond value, but you can only parse a String.
However, it seems you already have a millisecond value available, so only the formatting is required, which is via the print()
method:
DateTimeFormatter dtf_out = DateTimeFormat.forPattern("yyyy-MM-dd");
String b = dtf_out.print(resultsJs.get(0).getTimestamp().getTime());