new File(url).lastModified() returns a long equal to the number of milliseconds since the epoch, which is GMT-based.
What is a simple way to convert this to a String representing system-local date/time?
If you really need to see an attempt from me here it is but it's a terrible mess and it's wrong anyway:
LocalDateTime.ofEpochSecond(new File(url).lastModified()/1000,0,ZoneOffset.UTC).atZone(ZoneId.of("UTC")).format(DateTimeFormatter.ofLocalizedDateTime(FormatStyle.LONG))
Beyond LocalDateTime I just have no idea how the time API works.
To get the last modified time of a file, you should use Java NIO.2 API, which directly resolves your problem:
FileTime fileTime = Files.getLastModifiedTime(Paths.get(url));
System.out.println(fileTime); // will print date time in "YYYY-MM-DDThh:mm:ss[.s+]Z" format
If you want to access other properties (like last access time, creation time), you can read the basic attributes of a path with Files.readAttributes(path, BasicFileAttributes.class).