Is there any way to convert ZoneId to ZoneOffset in Java 8?

Issue

I have an epoch second and a zoneId (see method1 below).

It can be convert to LocalDateTime with system default zoneId, but I don't find the way to convert epoch second to LocalDateTime (see method2 below), because there is no ZoneOffset.systemDefault. I think it's obscure.

import java.time.{Instant, LocalDateTime, ZoneId, ZoneOffset}

val epochSecond = System.currentTimeMillis() / 1000

// method1
LocalDateTime.ofInstant(Instant.ofEpochSecond(epochSecond), ZoneId.systemDefault())

// method2
LocalDateTime.ofEpochSecond(epochSecond, 0, ZoneOffset.MAX)

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NOTE

The source code presented above is Scala.

Solution

Here is how you can get ZoneOffset from ZoneId:

Instant instant = Instant.now(); //can be LocalDateTime
ZoneId systemZone = ZoneId.systemDefault(); // my timezone
ZoneOffset currentOffsetForMyZone = systemZone.getRules().getOffset(instant);

NB: ZoneId can have different offset depending on point in time and the history of the particular place. So choosing different Instants would result in different offsets.

NB2: ZoneId.of() can return a ZoneOffset instead of ZoneId if UTC+3/GMT+2/etc is passed as opposed to a time zone like Africa/Cairo. So if UTC/GMT offsets are passed then historical/geographical/daylight-saving information of the Instant won't be taken into account - you'll simply work with the specified offset.

Answered By - Stanislav Bashkyrtsev