datetime - Java 8 localdatetime vs Date

Question

I have a string 2017-07-31T01:01:00-07:00 and I am trying to parse it to date and in CST Timezone. I am getting different results when i parse this string using Date and Java 8 ZonedDateTime. I am not getting why this is happening and what I am doing wrong.

    String dateStr = "2017-07-31T01:01:00-07:00";
    LocalDateTime time = null;
    SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss-hh");

    String[] dateArray = dateStr.split("-");
    String[] timeZones = TimeZone
            .getAvailableIDs(TimeZone.getTimeZone("GMT-" + dateArray[dateArray.length - 1]).getRawOffset());
    format.setTimeZone(TimeZone.getTimeZone(timeZones[0]));
    Date dateObj = null;
    try {
        dateObj = format.parse(dateStr);
    } catch (ParseException e) {
        e.printStackTrace();
    }
    time = dateObj.toInstant().atZone(TimeZone.getTimeZone("CST").toZoneId()).toLocalDateTime();
    ZonedDateTime time2 = ZonedDateTime.parse(dateStr).toInstant().atZone(TimeZone.getTimeZone("CST").toZoneId());
    System.out.println(time);
    System.out.println(time2.toLocalDateTime());

Answer 1

You should not parse the timezone offset yourself. Just use the X pattern:

new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssX")

You should not use timezone CST, since that is ambiguous (Central Standard Time, China Standard Time, Cuba Standard Time). Use America/Chicago (I assume that's what you meant).

So, to parse the date string with old and new APIs:

String dateStr = "2017-07-31T01:01:00-07:00";

// Using Date
SimpleDateFormat parseFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssX");
Date date = parseFormat.parse(dateStr);

SimpleDateFormat printFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm");
printFormat.setTimeZone(TimeZone.getTimeZone("America/Chicago"));
System.out.println(printFormat.format(date));

// Using ZonedDateTime
ZonedDateTime zonedDateTime = ZonedDateTime.parse(dateStr);
zonedDateTime = zonedDateTime.withZoneSameInstant(ZoneId.of("America/Chicago"));
System.out.println(zonedDateTime.toLocalDateTime());

Output

2017-07-31T03:01
2017-07-31T03:01

If you want to see the time zone, you can do this:

String dateStr = "2017-07-31T01:01:00-07:00";

// Using Date
SimpleDateFormat parseFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssX");
Date date = parseFormat.parse(dateStr);

SimpleDateFormat printFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm XXX z");
printFormat.setTimeZone(TimeZone.getTimeZone("America/Chicago"));
System.out.println(printFormat.format(date));

// Using ZonedDateTime
ZonedDateTime zonedDateTime = ZonedDateTime.parse(dateStr);

DateTimeFormatter printFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm XXX z")
                                                    .withZone(ZoneId.of("America/Chicago"));
System.out.println(zonedDateTime.format(printFormatter));

Output

2017-07-31T03:01 -05:00 CDT
2017-07-31T03:01 -05:00 CDT

Note how the first example changes the ZonedDateTime timezone, converts it to a LocalDateTime, then prints that without a formatter, while in the second example the DateTimeFormatter is setup to format the value in a specific timezone, similar to how the SimpleDateFormat is doing it. Just different ways of accomplishing the same result.