Date Formatting In Java With Time Zone

The Problem
GAE's system timezone is UTC and I need print date using the specified timezone and format.

The Solution
We can call DateFormat.setTimeZone to set it's timezone:

  public static String formatDate(Date date, String format, TimeZone tz) {
    DateFormat df = new SimpleDateFormat(format);
    df.setTimeZone(tz);
    return df.format(date);
  }

To get get all available time zone ids: System.out.println(Arrays.toString(TimeZone.getAvailableIDs()));
The test code is like below:

  @Test
  public void testDfWithTZ() {
    // To get all available time zone ids:
    System.out.println(Arrays.toString(TimeZone.getAvailableIDs()));
    Date date = new Date();
    String format = "yyyy-MM-dd hh:mm:ss";
    TimeZone tz = TimeZone.getTimeZone("US/Eastern");
    String dateStr = formatDate(date, format, tz);
    System.out.println(dateStr);
    
    tz = TimeZone.getTimeZone("Asia/Shanghai");
    dateStr = formatDate(date, format, tz);
    System.out.println(dateStr);
    
    // Get system's default timezone
    tz = TimeZone.getDefault();
    dateStr = formatDate(date, format, tz);
    System.out.println(dateStr);
  }