Runtime's session timezone possible problem

Main.scala

package classloaders

import java.time._
import java.util.TimeZone
import java.util.concurrent.TimeUnit

import scala.concurrent.duration.Duration

object Main {

  private val dt = LocalDateTime.of(1900, 1, 2, 3, 4, 5)

  def main(args: Array[String]): Unit = {
    pprint(dt.atZone(ZoneOffset.UTC))
    pprint(dt.atZone(ZoneId.systemDefault()))
    pprint(dt.atZone(ZoneId.of("GMT+02:00")))
    pprint(dt.atZone(simulateRuntime("GMT+02:00"))) // this is expected to yield the same result as the former
  }
  
  private def pprint(time: Any): Unit = {
    def diff(instant: Instant): String = {
      ((instant.toEpochMilli / 1000) - (dt.atZone(ZoneOffset.UTC).toInstant.toEpochMilli / 1000)).toString
    }
    println(time match {
      case zdt: ZonedDateTime => s"[${diff(zdt.toInstant)}] $zdt"
      case instant: Instant => s"[${diff(instant)}] $instant"
    })
  }

  private def simulateRuntime(zoneId: String): ZoneId = {
    val originalTimeZone = TimeZone.getTimeZone(zoneId)
    val session = new RuntimeSession()
    session.setTimeZone(getJavascriptDatetimeOffset(originalTimeZone))
    session.getTimeZone.toZoneId
  }

  class RuntimeSession { // What we do in runtime
    var timeZone: TimeZone = _
    def getTimeZone: TimeZone = timeZone
    def setTimeZone(offset: Int): Unit = timeZone = calculateTimeZone(offset)
    def calculateTimeZone(timezoneOffset: Int): TimeZone = {
      val zone = TimeZone.getTimeZone("UTC")
      zone.setRawOffset(timezoneOffset * -1 * 60 * 1000) //Javascript returns timezone offset in negative minutes (so -120 for UTC+2)
      val roundedOffsetInHours = Math.round(timezoneOffset / 60f * -1)
      zone.setID("Etc/GMT" + (if (roundedOffsetInHours < 0) roundedOffsetInHours else "+" + roundedOffsetInHours))
      // the problem gets solved if we change "Etc/GMT" to "GMT"
      zone
    }
  }

  def getJavascriptDatetimeOffset(timeZone: TimeZone): Int = {
    // scale it to javascript's timezone offset (see javascript's )
    // (e.g. javascript "new Date().getTimezoneOffset()" returns -120 for CET)
    timeZone.getRawOffset / (-1 * 60 * 1000)
  }

}

output.txt

[0]     1900-01-02T03:04:05Z
[-1172] 1900-01-02T03:04:05+00:19:32[Europe/Amsterdam]
[-7200] 1900-01-02T03:04:05+02:00[GMT+02:00]
[7200]  1900-01-02T03:04:05-02:00[Etc/GMT+2] // why the zone offset here is -02:00 ?