How to custom format a FileTime

Personally I find the error message "Unsupported field: Year" misleading. The real cause is missing timezone. This information is needed to help the formatter to internally convert the given instant to a human-time-representation. Solution: Supply the timezone. Then formatting or parsing an Instant is supported - in contrast to the answer of @flo.

Printing:

String s = 
  DateTimeFormatter.ofPattern("uuuu-MMM-dd HH:mm:ss", Locale.ENGLISH)
    .withZone(ZoneId.systemDefault())
    .format(Instant.now());
System.out.println(s); // 2015-Oct-30 15:22:32

Parsing:

The reverse procedure - parsing - does unfortunately not work the same direct way because the format engine of java.time is designed such that the formatter only returns a raw TemporalAccessor which needs to be converted to the real required type. Example:

Instant instant =
  Instant.from(
    DateTimeFormatter.ofPattern("uuuu-MMM-dd HH:mm:ss", Locale.ENGLISH)
    .withZone(ZoneId.systemDefault())
    .parse("2015-Oct-30 15:22:32"));
System.out.println("=>" + instant); // 2015-10-30T14:22:32Z

If the input to be parsed contains a timezone offset or an identifier then you can modify the pattern (symbols x, X, z, Z, VV etc.) and leave out the call to withZone(...), and in case of offsets - you really should leave out that call because otherwise the formatter will not use the timezone offset of your input but the supplied one zone (a pitfall I observed in my own tests).

You cannot format an Instant using a DateTimeFormatter instance querying the year.

An Instant is representing a single point on the time line. That's why it is not possible to give a correct/unique answer to the question "what's the year/day/time?". It depends on where on the world the question is asked: In New York it differs from Sidney. But your DateTimeFormatter is asking exactly this question. And that is why you get an UnsupportedTemporalTypeException.

You have to convert the Instance to a LocalDateTime at least:

System.out.println(timestampFormatter.format(
    LocalDateTime.ofInstant(fileTime.toInstant(), ZoneId.systemDefault()));

Formatting an Instant requires a time-zone. This can be achieved using withZone(ZoneId):

String s = DateTimeFormatter.ofPattern("uuuu-MMM-dd HH:mm:ss")
                 .withZone(ZoneId.systemDefault())
                 .format(fileTime.toInstant());