Java Time: Create your own Timezone

Question

I'm trying to migrate an old library to use the "new" Java time API and struggle on one point.

Context

I developed quite some time now an application to handle charts in the banking industry. So you have a continuous flow of prices coming and you aggregate them by 1 day candles (for instance) with an open, high, low and close.

The main issue came with the fact that the charts need to display candles only when the market is open. For forex, this means from Sunday 17:00 NY till Friday 17:00 NY. So the day starts at 17:00 NY time and your candles need to reflect that even if you live in Geneva.

The way I did this with the old "Java Time" API was to create my own timezone (see code below)

public class RelativeTimezone extends TimeZone {
  private final TimeZone zone;
  private final int addedTime;

  public RelativeTimezone(String id, TimeZone zone, int addedTime) {
    this.zone = zone;
    this.addedTime = addedTime;
    setID(id);
  }

  public final int getAddedTime() {
    return this.addedTime;
  }

  @Override
  public int getRawOffset() {
    return this.zone.getRawOffset() + this.addedTime;
  }

  @Override
  public void setRawOffset(int offsetMillis) {
    this.zone.setRawOffset(offsetMillis - this.addedTime);
  }

  // Other overriding methods that simply forward to "this.zone"
  // ...
}

By calling:

TimeZone forexTimezone = new RelativeTimezone("NY+7", 
      TimeZone.getTimeZone("America/New_York"), 7 * 3_600_000L)

I get a Timezone where monday midnight is actually sunday 17:00 NY. This greatly helps with all the computations to know which day it is. And I automatically get the Daylight saving times correctly.

Problem:

I did not find a way to do the same with the "new" time API. The classes are either final or package protected so I'm unable to create my own TimeZone. I tried to find everywhere if someone had done something like that in the past but didn't find anything.

I found that I could create a new ZoneRules to make my own ZoneId, but impossible to use the initial ZoneRules (for NY) to get the initial parameters to feed it (why isn't it an interface that I could "decorate"). The only solution that I see is to use "reflexion" to read the NY ZoneRules fields.

Would there be a way to do this cleanly ?

Of course as long as the "old" API is there I can still use it, but for how long.