If you are using Java 8, you should not use java.util.Date in the first place (unless you receive the Date object from a library that you have no control over).
In any case, you can convert a Date to a java.time.Instant using:
Date date = ...;
Instant instant = date.toInstant();
Since you are only interested in the date and time, without timezone information (I assume everything is UTC), you can convert that instant to a LocalDateTime object:
LocalDateTime ldt = instant.atOffset(ZoneOffset.UTC).toLocalDateTime();
Finally you can print it with:
DateTimeFormatter fmt = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss");
System.out.println(ldt.format(fmt));
Or use the predefined formatter, DateTimeFormatter.ISO_LOCAL_DATE_TIME.
System.out.println(ldt.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
Note that if you don’t provide a formatter, calling ldt.toString gives output in standard ISO 8601 format (including milliseconds) – that may be acceptable for you.