Determine daylight saving time for US ZoneIds in Java

Question

I need to find out whether an Instant is in DST or not. I would expect it to work like that:

ZoneId z = ZoneId.of("America/New_York");
ZoneRules zr = z.getRules();
TimeZone tz = TimeZone.getTimeZone(z);

Instant summer = ZonedDateTime.of(2018, 5, 1, 2, 2, 2, 2, z).toInstant();
Instant winter = ZonedDateTime.of(2018, 11, 1, 2, 2, 2, 2, z).toInstant();

System.out.println("ZoneRules winter: " + zr.isDaylightSavings(winter));
System.out.println("ZoneRules summer: " + zr.isDaylightSavings(summer));
System.out.println("TimeZone winter: " + tz.inDaylightTime(Date.from(winter)));
System.out.println("TimeZone summer: " + tz.inDaylightTime(Date.from(summer)));

but my output for "America/New_York" and other US timezones is always:

ZoneRules winter: true
ZoneRules summer: true
TimeZone winter: true
TimeZone summer: true

ZoneRules.getOffset() also always returns the same value.

However, I don't see the same behaviour when working with European timezones. With ZoneId.of("Europe/London") or ZoneId.of("Europe/Amsterdam") I am getting the expected result:

ZoneRules winter: false
ZoneRules summer: true
TimeZone winter: false
TimeZone summer: true

What am I missing?

Here is some example code on ideone.

Answer 1

Your winter variable is equivalent to November 1^st 2018 in New York. But in 2018, in New York timezone, DST ends in November 4^th, so the winter variable is still in DST.

You can check when the next transition will be using the nextTransition method:

System.out.println(zr.nextTransition(winter));

This will print:

Transition[Overlap at 2018-11-04T02:00-04:00 to -05:00]

Meaning that in November 4^th, at 2 AM, clocks will be set 1 hour back (the "fall back" part of "spring forward/fall back rule"), and the offset will be changed from -04:00 to -05:00.

You can check that by creating the equivalent Instant:

// November 4th, at 02:02 AM (after DST transition)
Instant winter = ZonedDateTime.of(2018, 11, 4, 2, 2, 2, 2, z).toInstant();
System.out.println(zr.isDaylightSavings(winter)); // false

---

Another way to check the transition dates is to loop through the transtions provided by the ZoneRules:

zr.getTransitions().forEach(System.out::println);

Some zones don't have specific transitions for each year (that's the case of America/New_York), but instead they have transition rules (such as "DST starts at the third Sunday of October"), so if you don't find a transition for a specific year using the getTransitions() method, you can check the transition rules:

zr.getTransitionRules().forEach(r -> System.out.println(r.createTransition(2018)));

For America/New_York, the output is:

Transition[Gap at 2018-03-11T02:00-05:00 to -04:00]
Transition[Overlap at 2018-11-04T02:00-04:00 to -05:00]