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Programmer Certification (OCPJP)

Enthuware. Offset difference between OffsetDateTime and ZonedDateTime for the same place - Why?

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from Enthuware mock exam #9 enthuware.ocpjp.v17.2.3786

Given :
1. The time zone of America/New York is normally 5 hours behind UTC.
2. Day light saving is ON during June in New York. (This means, clocks in New York are 1 hour ahead of the regular time that would have been shown when day light saving is OFF.)

What will the following code print when compiled and run?

As I understand, OffsetDateTime is a date-time with an offset.
Stepping through the code in the debugger, I am surprised to find that the offset for New York: nyOdt = -5 as expected, but the offset for nyZdt is -4.
Why this difference?
Please see screenshot of debugger window for values.

date-time.png

Tim Holloway

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I would venture that that reflects the difference between Standard Time and Daylight Savings Time.

The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.

You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.

Anil Philip

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Tim Holloway wrote:I would venture that that reflects the difference between Standard Time and Daylight Savings Time.

thanks. Are you saying that OffsetDateTime is the time when DST is OFF?

Campbell Ritchie

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Have you read the documentation for ZonedDateTime and OffsetDateTime? That should explain the difference.

Ron McLeod

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Anil Philip wrote:Are you saying that OffsetDateTime is the time when DST is OFF?

OffsetDateTime has a fixed offset, and does not support the concept of time zone where the offset may vary during the year due to daylight saving time.

Anil Philip

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Ron McLeod wrote:

Anil Philip wrote:Are you saying that OffsetDateTime is the time when DST is OFF?

OffsetDateTime has a fixed offset, and does not support the concept of time zone where the offset may vary during the year due to daylight saving time.

thanks. I could not understand the explanation from the API docs.
I think OffsetDateTime is what the time would be around the world, if DST were to be removed or abandoned.

Stephan van Hulst

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OffsetDateTime is literally just a local date and time plus a fixed offset. It is not bound to a particular region or culture.

ZonedDateTime is much more complex. It doesn't have a fixed offset, but rather "rules" that may modify the local time in any kind of way, based on the laws and customs in the associated time zone.

The rules for a particular time zone may or may not include the notion of Daylight Savings, but may also include very different rules such as when Western Samoa moved from UTC-11 to UTC+13 in 2011.

Anil Philip

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Stephan van Hulst wrote:OffsetDateTime is literally just a local date and time plus a fixed offset. It is not bound to a particular region or culture.
ZonedDateTime is much more complex. It doesn't have a fixed offset, but rather "rules" that may modify the local time in any kind of way, based on the laws and customs in the associated time zone.

I think I understand it now. In the example above, the ny offset date-time, nyOdt, had the offset of -5.
Then the zoned date-time, nyZdt, was created from this. This zoned date-time recognized that DST was ON and reduced the offset to -4.
Thank you for helping me understand!

Tim Holloway

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Anil Philip wrote:
I think OffsetDateTime is what the time would be around the world, if DST were to be removed or abandoned.

Ironically, a number of US States want to abandon Standard Time in favor of permanent DST.

They voted on it several years ago, but cannot implement it because the US Congress has more important matters to attend to (snicker).

Anil Philip

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Tim Holloway wrote:Ironically, a number of US States want to abandon Standard Time in favor of permanent DST.

Would that be going forward in Spring and staying there? Or fall back in Fall and remaining there.

Tim Holloway

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Anil Philip wrote:

Tim Holloway wrote:Ironically, a number of US States want to abandon Standard Time in favor of permanent DST.

Would that be going forward in Spring and staying there? Or fall back in Fall and remaining there.

"Fall" back returns you to standard time. Basically, we no longer would fall back, then. Because it would be silly to flip and flop at one time.

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