Joda-Time, Daylight Saving Time change and date time parsing

I have the following problem using Joda-Time for parsing and producing date and time around Daylight Saving Time (DST) hours. Here is an example (please, note that March 30th 2008 is Daylight Saving change in Italy):

DateTimeFormatter dtf = DateTimeFormat.forPattern("dd/MM/yyyy HH:mm:ss");
DateTime x = dtf.parseDateTime("30/03/2008 03:00:00");
int h = x.getHourOfDay();
System.out.println(h);
System.out.println(x.toString("dd/MM/yyyy HH:mm:ss"));
DateTime y = x.toDateMidnight().toDateTime().plusHours(h);
System.out.println(y.getHourOfDay());
System.out.println(y.toString("dd/MM/yyyy HH:mm:ss"));

I get th开发者_如何学Goe following output:

3
30/03/2008 03:00:00
4
30/03/2008 04:00:00

When i parse hour I get hour is 3. In my data structure I save the day storing midnight time, and then I have some value for each hour of the day (0-23). Then, when I write out the date, I re-compute the full date time making midnight plus hour. When I sum 3 hours to my midnight I get 04:00:00! And if I parse it again, I get hour 4!

Where is my mistake? Is there some way to get hour 2 when I parse or get hour three when I print out?

I have also tried to build output by hand:

String.format("%s %02d:00:00", date.toString("dd/MM/yyyy"), h);

but in this case for hour 2, I produce 30/03/2008 02:00:00 which is not a valid date (since hour 2 does not exist) and cannot be parsed any more.

Thank you in advance for your help. Filippo

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When I sum 3 hours to my midnight I get 04:00:00! And if I parse it again, I get hour 4! Where is my mistake?

You mentioned already that this date is exactly when the time changes. So there is no mistake. March 30, 2010 00:00 CEST (the timezone in Italy) is precisely speaking March 29, 2010 23:00 UTC. When you add 3 hours, you will get March 30, 2010 02:00 UTC. But this is post the moment, that we switch times (which happens on 01:00 UTC), so when you convert time to local timezone you get March 30, 04:00. That's correct behavior.

Is there some way to get hour 2 when I parse or get hour three when I print out?

No, because March 30, 2010 02:00 CEST does not exist. Precisely at March 30, 2010 01:00 UTC we switch time from +1 hour to +2 hours versus UTC, so March 30, 2010 00:59 UTC is March 30, 2010: 01:59 CEST, but March 30, 2010 01:00 UTC become March 30, 2010 03:00 CEST. No 02:xx hour exist on that particular date.

BTW. In a week you can expect another "fun". Can you tell what date in UTC this refers to:

October 31, 2010 02:15 CEST ?

Well, the funny part is, we do not know. It could be either 0ctober 31, 2010 00:15 UTC (before actual time switch) or October 31, 2010 01:15 UTC (after the switch).

That's exactly why you should always store date and times in relation to UTC and convert them to local time zone before displaying, otherwise you risk an ambiguity.

HTH.

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The data structure you are saving your data is not very optimal for the days with daylight saving time. Your day in this particular day should only have 23 hours.

If you do:

    DateTimeFormatter dtf = DateTimeFormat.forPattern("dd/MM/yyyy HH:mm:ss").withLocale(Locale.US);
    DateTime x = dtf.parseDateTime("30/03/2008 00:00:00");

    DateTimeFormatter parser = DateTimeFormat.fullDateTime();
    System.out.println("Start:"+parser.print(x));

    DateTime y = x.plusHours(4);

    System.out.println("After add of 4:"+parser.print(y));

You get the expected result, that the time is 05:00.

I recommend that you change the way you store your day and use a date. If not, you must handle daylight saving time when storing the hour of day.

You might do something like this: In the case where we move the time forward one hour, as this case, you must store 4 and not 5 as the time for 5. And when you calculate the time, you should use the plusHours() method to get the actual time. I think you might get away with something like:

public class DateTest {
    private static final int HOUR_TO_TEST = 2;

  public static void main(String[] args) {
    DateTimeFormatter dtf = DateTimeFormat.forPattern("dd/MM/yyyy HH:mm:ss");
    DateTime startOfDay = dtf.parseDateTime("30/03/2008 00:00:00");

    /* Obtained from new DateTime() in code in practice */
    DateTime actualTimeWhenStoring = startOfDay.plusHours(HOUR_TO_TEST);

    int hourOfDay = actualTimeWhenStoring.getHourOfDay();
    int hourOffset = startOfDay.plusHours(hourOfDay).getHourOfDay();

    System.out.println("Hour of day:" + hourOfDay);
    System.out.println("Offset hour:" + hourOffset);

    int timeToSave = hourOfDay;
    if (hourOffset != hourOfDay) {
        timeToSave = (hourOfDay + (hourOfDay - hourOffset));
    }
    System.out.println("Time to save:" + timeToSave);

    /* When obtaining from db: */
    DateTime recalculatedTime = startOfDay.plusHours(timeToSave);

    System.out.println("Hour of time 'read' from db:" + recalculatedTime.getHourOfDay());
  }
}

...or basicly something like that. I'd write a test for it if you choose for going down this route. You can change the HOUR_TO_TEST to see that it moves passed the daylight saving time.

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Building on the correct answers by Paweł Dyda & Knubo…

ISO 8601 For String Format

You should never store (serialize) a date-time as a string in the format you mentioned: "30/03/2008 03:00:00". Problems:

• Omitted time zone.
• Day, Month, Year order is ambiguous.
• Should have been translated to UTC time.

If you must serialize a date-time value to text, use a reliable format. The obvious choice is the ISO 8601 standard format. Even better is converting the local time to UTC (Zulu) time zone and then out to ISO 8601 format. Like this: 2013-11-01T04:48:53.044Z

No Midnight

The midnight methods in Joda-Time are deprecated in favor of the Joda-Time method withTimeAtStartOfDay() (see doc). Some days do not have a midnight.

Example Code in Joda-Time 2.3

Some comments about this source code:

    // © 2013 Basil Bourque. This source code may be used freely forevery by anyone taking full responsibility for doing so.

    // Joda-Time - The popular alternative to Sun/Oracle's notoriously bad date, time, and calendar classes bundled with Java 7 and earlier.
    // http://www.joda.org/joda-time/

    // Joda-Time will become outmoded by the JSR 310 Date and Time API introduced in Java 8.
    // JSR 310 was inspired by Joda-Time but is not directly based on it.
    // http://jcp.org/en/jsr/detail?id=310

    // By default, Joda-Time produces strings in the standard ISO 8601 format.
    // https://en.wikipedia.org/wiki/ISO_8601

Example showing 23 hours in the day of DST (Daylight Saving Time) in Rome Italy, while the day after has 24 hours. Note that the time zone (for Rome) is specified.

    // Time Zone list: http://joda-time.sourceforge.net/timezones.html
    org.joda.time.DateTimeZone romeTimeZone = org.joda.time.DateTimeZone.forID("Europe/Rome");
    org.joda.time.DateTime dayOfDstChange = new org.joda.time.DateTime( 2008, 3, 30, 0, 0, romeTimeZone ) ; // Day when DST
    org.joda.time.DateTime dayAfter = dayOfDstChange.plusDays(1);

    // How many hours in this day? Should be 23 rather than 24 on day of Daylight Saving Time "springing ahead" to lose one hour.
    org.joda.time.Hours hoursObjectForDay = org.joda.time.Hours.hoursBetween(dayOfDstChange.withTimeAtStartOfDay(), dayAfter.withTimeAtStartOfDay());
    System.out.println( "Expect 23 hours, got: " + hoursObjectForDay.getHours() ); // Extract an int from object.

    // What time is 3 hours after midnight on day of DST change?
    org.joda.time.DateTime threeHoursAfterMidnightOnDayOfDst = dayOfDstChange.withTimeAtStartOfDay().plusHours(3);
    System.out.println( "Expect 4 AM (04:00) for threeHoursAfterMidnightOnDayOfDst: " + threeHoursAfterMidnightOnDayOfDst );

    // What time is 3 hours after midnight on day _after_ DST change?
    org.joda.time.DateTime threeHoursAfterMidnightOnDayAfterDst = dayAfter.withTimeAtStartOfDay().plusHours(3);
    System.out.println( "Expect 3 AM (03:00) for threeHoursAfterMidnightOnDayAfterDst: " + threeHoursAfterMidnightOnDayAfterDst );

Example of storing a date-time by first translating to UTC. Then upon restoring the date-time object, adjust to the desired time zone.

    // Serialize DateTime object to text.
    org.joda.time.DateTimeZone romeTimeZone = org.joda.time.DateTimeZone.forID("Europe/Rome");
    org.joda.time.DateTime dayOfDstChangeAtThreeHoursAfterMidnight = new org.joda.time.DateTime( 2008, 3, 30, 0, 0, romeTimeZone ).withTimeAtStartOfDay().plusHours(3);
    System.out.println("dayOfDstChangeAtThreeHoursAfterMidnight: " + dayOfDstChangeAtThreeHoursAfterMidnight);
    // Usually best to first change to UTC (Zulu) time when serializing.
    String dateTimeSerialized = dayOfDstChangeAtThreeHoursAfterMidnight.toDateTime( org.joda.time.DateTimeZone.UTC ).toString();
    System.out.println( "dateTimeBeingSerialized: " + dateTimeSerialized );
    // Restore
    org.joda.time.DateTime restoredDateTime = org.joda.time.DateTime.parse( dateTimeSerialized );
    System.out.println( "restoredDateTime: " + restoredDateTime );
    // Adjust to Rome Italy time zone.
    org.joda.time.DateTime restoredDateTimeAdjustedToRomeItaly = restoredDateTime.toDateTime(romeTimeZone);
    System.out.println( "restoredDateTimeAdjustedToRomeItaly: " + restoredDateTimeAdjustedToRomeItaly );

When run:

dayOfDstChangeAtThreeHoursAfterMidnight: 2008-03-30T04:00:00.000+02:00
dateTimeBeingSerialized: 2008-03-30T02:00:00.000Z
restoredDateTime: 2008-03-30T02:00:00.000Z
restoredDateTimeAdjustedToRomeItaly: 2008-03-30T04:00:00.000+02:00