Joda Time to Java Time Migration 在迁移时显示不同的结果

Question

Given the following code

public static void main(String[] args) {
     org.joda.time.format.DateTimeFormatter _timestampFomatNYCJoda = org.joda.time.format.DateTimeFormat.forPattern("yyyyMMdd HHmmss.SSS").withZone(DateTimeZone.forID("America/New_York"));
     DateTimeFormatter _timestampFomatNYC = DateTimeFormatter.ofPattern("yyyyMMdd HHmmss.SSS").withZone(ZoneId.of("America/New_York"));
     LocalDateTime localDateTime = LocalDateTime.now();
     org.joda.time.LocalDateTime jodaLocalDateTime = new org.joda.time.LocalDateTime();
     System.out.println("System Time " + new Date());
     System.out.println("Java Version " +  localDateTime.format(_timestampFomatNYC));
     System.out.println("Joda Version " +  _timestampFomatNYCJoda.print(jodaLocalDateTime.toDateTime(DateTimeZone.UTC)));
} 

Why does the Java Version and Joda Version dont match ? I am running this on IST clock.

Below is the output

System Time Fri Mar 27 17:01:33 IST 2020
Java Version 20200327 170133.933
Joda Version 20200327 130133.938

Answer 1

I can reproduce your results. I can also explain them. Joda-Time and java.time have been designed to behave differently in this case. Let's look at them in turn.

Joda-Time

In Joda-Time DateTimeFormatter.withZone() gives you a formatter with an override zone , that is, a zone that will always be used for formatting dates and times. In other words, any date and time will be converted to this zone for printing. The documentation says:

When printing, this zone will be used in preference to the zone from the datetime that would otherwise be used.

When you do new org.joda.time.LocalDateTime() , you are getting a LocalDateTime representing the current date and time in your default time zone. The Local in some class names means without time zone or offset from UTC . I figure that you must have got a value equal to 2020-03-27T17:01:33.938 .

Apparently what happens when you format a LocalDateTime with a formatter with an override zone, is that the formatter assumes that your LocalDateTime is in UTC (which yours isn't) and converts it from there, in your case to America/New_York time zone. Since summer time (DST) is in effect in New York, the offset is -04:00, so 17:01 becomes 13:01.

This is the wrong result. When the time is 17:01 in your time zone, it is not 17:01 UTC, so the conversion is based on a false premise. It is also not 13:01 in New York, so the converted result is telling a lie.

java.time

With java.time setting an override zone on a formatter works similarly for formatting, but with a difference that matters here: the override zone is only used when printing a date-time object that identifies an instant (a point in time). From the docs:

When formatting, if the temporal object contains an instant, then it will be converted to a zoned date-time using the override zone. Whether the temporal is an instant is determined by querying the INSTANT_SECONDS field. If the input has a chronology then it will be retained unless overridden. If the input does not have a chronology, such as Instant, then the ISO chronology will be used.

… In all other cases, the override zone is added to the temporal, replacing any previous zone, but without changing the date/time.

Again LocalDateTime.now() gives you the current date and time of day (a few milliseconds earlier than the query through Joda-Time), 2020-03-27T17:01:33.933 . Local still means without offset or time zone.

Because your LocalDateTIme hasn't got offset or time zone, it cannot identify an unambigous point in time, an instant. Therefore when formatting it neither the date nor the time of day is changed. And since your format pattern contains no time zone or offset, none is printed. So you just get the date and time in your time zone (not in New York), 20200327 170133.933 .

To get the date and time in New York time zone

    DateTimeFormatter timestampFormat
            = DateTimeFormatter.ofPattern("yyyyMMdd HHmmss.SSS");
    ZonedDateTime timeInNy = ZonedDateTime.now(ZoneId.of("America/New_York"));
    System.out.println(timeInNy.format(timestampFormat));

When I ran this code just now, the output was:

20200327 122359.683

Documentation links

• Joda-Time DateTimeFormatter.withZone()
• java.time DateTimeFormatter.withZone()