About Question enthuware.ocpjp.v8.2.1750 :

[ramlax]

So the question is:
"Given that daylight Savings Time starts on March 8th at 2 AM in US/Eastern time zone. (As a result, 2 AM becomes 3 AM.)"

Here we have March/08 02:00AM it means you have 03:00.
Here we have March/08 03:00AM it means you have 03:00.

Can someone explain to me why is the result of ChronoUnit.HOURS.between function here 0, considering the analogy in the
from the question viewtopic.php?f=2&t=3659?

Apparently the time in DST start (March) is moved ahead to 3 AM in contrast with DST end time (November) which is not moved back to 1 AM, or I misunderstood something?

Code: Select all

LocalDateTime ld1 = LocalDateTime.of(2015, Month.NOVEMBER, 1, 2, 0);    // 2015-11-01T02:00
ZonedDateTime zd1 = ZonedDateTime.of(ld1, ZoneId.of("US/Eastern"));     // 2015-11-01T02:00-05:00[US/Eastern
LocalDateTime lod1 = LocalDateTime.of(2015, Month.MARCH, 8, 2, 0);       // 2015-03-08T02:00
ZonedDateTime zod1 = ZonedDateTime.of(lod1, ZoneId.of("US/Eastern")); // 2015-03-08T03:00-04:00[US/Eastern] 

Thank you for your help.

[admin]

It is the same logic. How many hours do you need to add to reach 3AM from 2AM. Usually, 1. But on this particular day, you need to add 0 hours.

[ramlax]

Apparently my question was not precise. Let's forget about this concrete exercise at all.

I just want to understand the following logic.
2015 ---> Sun, 8 Mar, 02:00 --- EST → EDT +1 hour (DST start)
2015 ---> Sun, 1 Nov, 02:00 --- EDT → EST -1 hour (DST end)

Let the first example be DST start, at 2:00 the time is changed for +1 hour. This is quite easy to understand.

Code: Select all

LocalDateTime lod1 = LocalDateTime.of(2015, Month.MARCH, 8, 2, 0); // 2015-03-08T02:00
ZonedDateTime zod1 = ZonedDateTime.of(lod1, ZoneId.of("US/Eastern"));// 2015-03-08T03:00-04:00[US/Eastern] 

From the example above it is obvious when LocalDateTime object is applied to the ZoneDateTime with specific zone, the +1 hour is added automatically. The time has changed it can be seen from the print.

According to this functioning one would expect the same goes for the DST end. But apparently it is not.
So at the 1st of November right at 2:00 the time should be changed for -1 hour.

Code: Select all

LocalDateTime ld1 = LocalDateTime.of(2015, Month.NOVEMBER, 1, 2, 0);    // 2015-11-01T02:00
ZonedDateTime zd1 = ZonedDateTime.of(ld1, ZoneId.of("US/Eastern"));     // 2015-11-01T02:00-05:00[US/Eastern

The time of zd1 is not changed.

Thank you for your explanation.

[admin]

You seem to have misunderstood the DST logic. When the daylight saving is ON, only during that duration (i.e. during those months) the zone time is 1 hour plus from the local time. When the day light is not ON, the local time and zone time are same. So there is no reason to expect a difference when the dst has ended.

Now, the issue of plus and minus comes only when you transition from dst to non-dst or vice versa by adding or subtracting hours.

HTH,
Paul.

[Wesley]

I got this question right but I still found it strange since on DST when the clocks go forward 2AM doesn't actually exist. The clock goes from 1:59.59 to 3:00.00. There is no 2AM. (assert mind == blown) So the question becomes how does Java interpret this phantom time? I haven't tested but according to the question it considers 2AM == 3AM.

If you don't understand the point I'm trying to make, think about this -> What if we set the time to 2:30AM and asked for the difference between 2:30 and 3:00 on that particular day? Without testing I have no idea if it will consider it 30 mins or also 0. My guess is it's programmed to take any time in the range 2:00 - 2:59 and bump it up to 3, so it'll also consider 2:30 AM to 3AM to be 0 minutes. Just a guess though.

[nowkarol]

We can state this question differently: which 1:00 will java create? We got two of theese. One with summer offset : -4 and one with normal (not DaylightSaving) : -5.

So we got 0:00 (offset -4), 1:00 (offset -4), 1:00 (offset -5).

Java by default will create date with offset prior to change (-4). With 2:00 there is no problem because this hour occures only with "new" offset (-5).

PS. This behavior can be changed by using withLaterOffsetAtOverlap on ZonedDateTime - called on zd2 in between method would result in -1.

I hope this thing will help, not confuse you more.