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

Question

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.

Answer 1

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.

Answer 2

There is no one-to-one mapping. A ZoneId defines a geographic extent in which a set of different ZoneOffsets is used over time. If the timezone uses daylight saving time, its ZoneOffset will be different between summer and winter.

Furthermore, the daylight saving time rules may have changed over time, so the ZoneOffset could be different for e.g. 13/10/2015 compared to 13/10/1980.

So you can only find the ZoneOffset for a ZoneId on a particular Instant.

See also https://en.wikipedia.org/wiki/Tz_database