UTC to local method in DateTimeZone of Joda-Time to Java 8 - java-8

Question

We are changing Joda-Time API's to Java 8 time API's. In Joda-Time I have used:
DateTimeZone.convertLocalToUTC(this.getMillis(), true);
DateTimeZone.convertUTCToLocal(long millis);
Can any one tell me equivalent methods in Java 8?
Edited
convertLocalToUTC
DateTimeZone dateTimeZone = DateTimeZone.getDefault();
DateTime jodadatetime = new DateTime();
long utcTime = dateTimeZone.convertLocalToUTC(jodadatetime .getMillis(), true);
System.out.println(jodadatetime);
DateTimeZone dateTimeZone1 = DateTimeZone.UTC;
System.out.println(new DateTime(utcTime, dateTimeZone1));
Output
2017-08-09T17:27:57.508+05:30
2017-08-09T06:27:57.508Z
ConvertUtcToLocal
long utctolocal = dateTimeZone.convertUTCToLocal(jodadatetime.getMillis());
System.out.println("utc to local : " + new DateTime(utctolocal, dateTimeZone1));
Output
2017-08-09T17:27:57.508Z

Answer 1

So, your original date is 2017-08-09T17:27:57.508+05:30, then you want 2 things:
convertLocalToUTC: get 2017-08-09T06:27:57.508Z. This is a little bit tricky:
The original date is 2017-08-09T17:27:57.508+05:30, which is equivalent in UTC to 2017-08-09T11:57:57.508Z. What this method does is to convert this to the same local date and time but at the Calcutta timezone, and then you're printing it in UTC. In short:
original date is 2017-08-09T17:27:57.508+05:30
in UTC, that's the same as 2017-08-09T11:57:57.508Z
convertLocalToUTC converts this to 2017-08-09T11:57:57.508+05:30 (same date and time, but in Calcutta timezone)
and that's the same as 2017-08-09T06:27:57.508Z
To do this in Java 8, you can do:
ZoneId zone = ZoneId.of("Asia/Calcutta");
// original date 2017-08-09T17:27:57.508+05:30
Instant i = OffsetDateTime.parse("2017-08-09T17:27:57.508+05:30")
// convert to UTC (2017-08-09T11:57:57.508Z)
.atZoneSameInstant(ZoneOffset.UTC)
// convert to same local time in Calcutta
.withZoneSameLocal(zone)
// back to UTC
.toInstant();
System.out.println(i.toEpochMilli() + "=" + i);
Output:
1502260077508=2017-08-09T06:27:57.508Z
convertUTCToLocal: get 2017-08-09T17:27:57.508Z - the same date (2017-08-09) and time (17:27:57.508) but in UTC.
It's similar:
ZoneId zone = ZoneId.of("Asia/Calcutta");
// original date 2017-08-09T17:27:57.508+05:30
ZonedDateTime z = OffsetDateTime.parse("2017-08-09T17:27:57.508+05:30")
// convert to a ZonedDateTime in Calcutta (2017-08-09T17:27:57.508+05:30[Asia/Calcutta])
.atZoneSameInstant(zone)
// convert to same local time in UTC
.withZoneSameLocal(ZoneOffset.UTC);
System.out.println(z.toInstant().toEpochMilli() + "=" + z);
Output:
1502299677508=2017-08-09T17:27:57.508Z
You can also get the dates from the millis value.
For case 1:
// millis for original joda date: jodadatetime.getMillis() (1502279877508 = 2017-08-09T17:27:57.508+05:30)
long millisFromJoda = 1502279877508L;
Instant instant = Instant.ofEpochMilli(millisFromJoda)
// convert to UTC (2017-08-09T11:57:57.508Z)
.atZone(ZoneOffset.UTC)
// convert to same local time in Calcutta
.withZoneSameLocal(zone)
// back to UTC
.toInstant();
System.out.println(instant.toEpochMilli() + "=" + instant);
Output:
1502260077508=2017-08-09T06:27:57.508Z
You can convert the Instant to another types if you want:
// convert to ZonedDateTime in UTC
ZonedDateTime zd = instant.atZone(ZoneOffset.UTC);
// convert to OffsetDateTime in UTC
OffsetDateTime odt = instant.atOffset(ZoneOffset.UTC);
Both will be 2017-08-09T06:27:57.508Z.
And for case 2:
ZonedDateTime zdt = Instant.ofEpochMilli(millisFromJoda)
// convert to a ZonedDateTime in Calcutta (2017-08-09T17:27:57.508+05:30[Asia/Calcutta])
.atZone(zone)
// convert to same local time in UTC
.withZoneSameLocal(ZoneOffset.UTC);
System.out.println(zdt.toInstant().toEpochMilli() + "=" + zdt);
Output:
1502299677508=2017-08-09T17:27:57.508Z

Answer 2

Here is some code snippet that may help you get started:
LocalDateTime yourLocalTime = ...
long utc = yourLocalTime.toInstant(ZoneOffset.UTC).toEpochMilli();
or as you used strict = true the following:
long utc = ZonedDateTime.ofStrict(yourLocalTime, ZoneOffset.UTC, ZoneId.of("Z")).toInstant().toEpochMilli();
Converting back is similar:
LocalDateTime yourUtcTime = ...
long localTimeInMillis = yourUtcTime.toInstant(OffsetDateTime.now().getOffset() /* or: yourLocalTime.getOffset() */).toEpochMilli();
If you don't need the millis, but want to work with the DateTime-classes instead, you may want to use ZonedDateTime instead?
Creating a LocalDateTime from millis could be done as follows:
LocalDateTime yourLocalDateTime = Instant.ofEpochMilli(millisAsLong).atZone(/* your desired zone here */).toLocalDateTime();
where zone id could be ZoneId.of("Z"), ZoneId.systemDefault(), etc.