ADDITIONAL SYSTEM INFORMATION :
Software:
    System Software Overview:
      System Version: macOS 14.5 (23F79)
      Kernel Version: Darwin 23.5.0
      Boot Volume: Macintosh HD
      Boot Mode: Normal
      Secure Virtual Memory: Enabled
      System Integrity Protection: Enabled
      Time since boot: 7 days, 44 minutes

Hardware:
    Hardware Overview:
      Model Name: MacBook Pro
      Model Identifier: Mac15,9
      Model Number: Z1CM000BRCZ/A
      Chip: Apple M3 Max
      Total Number of Cores: 16 (12 performance and 4 efficiency)
      Memory: 128 GB
      System Firmware Version: 10151.121.1
      OS Loader Version: 10151.121.1
      Activation Lock Status: Enabled

openjdk version "21.0.2" 2024-01-16
OpenJDK Runtime Environment (build 21.0.2+13-58)
OpenJDK 64-Bit Server VM (build 21.0.2+13-58, mixed mode, sharing)
VM settings:
    Max. Heap Size (Estimated): 29.50G
    Using VM: OpenJDK 64-Bit Server VM

Property settings:
    file.encoding = UTF-8
    file.separator = /
    ftp.nonProxyHosts = local|*.local|169.254/16|*.169.254/16
    http.nonProxyHosts = local|*.local|169.254/16|*.169.254/16
    java.class.path = .
        /usr/local/lib/antlr-4.13.1-complete.jar
    java.class.version = 65.0
    java.home = /Users/uuDev/.sdkman/candidates/java/21.0.2-open
    java.io.tmpdir = /var/folders/bm/ycht_2757mxczsk7tn5dzzh00000gp/T/
    java.library.path = /Users/uuDev/Library/Java/Extensions
        /Library/Java/Extensions
        /Network/Library/Java/Extensions
        /System/Library/Java/Extensions
        /usr/lib/java
        .
    java.runtime.name = OpenJDK Runtime Environment
    java.runtime.version = 21.0.2+13-58
    java.specification.name = Java Platform API Specification
    java.specification.vendor = Oracle Corporation
    java.specification.version = 21
    java.vendor = Oracle Corporation
    java.vendor.url = https://java.oracle.com/
    java.vendor.url.bug = https://bugreport.java.com/bugreport/
    java.version = 21.0.2
    java.version.date = 2024-01-16
    java.vm.compressedOopsMode = Non-zero based
    java.vm.info = mixed mode, sharing
    java.vm.name = OpenJDK 64-Bit Server VM
    java.vm.specification.name = Java Virtual Machine Specification
    java.vm.specification.vendor = Oracle Corporation
    java.vm.specification.version = 21
    java.vm.vendor = Oracle Corporation
    java.vm.version = 21.0.2+13-58
    jdk.debug = release
    line.separator = \n
    native.encoding = UTF-8
    os.arch = aarch64
    os.name = Mac OS X
    os.version = 14.5
    path.separator = :
    socksNonProxyHosts = local|*.local|169.254/16|*.169.254/16
    stderr.encoding = UTF-8
    stdout.encoding = UTF-8
    sun.arch.data.model = 64
    sun.boot.library.path = /Users/uuDev/.sdkman/candidates/java/21.0.2-open/lib
    sun.cpu.endian = little
    sun.io.unicode.encoding = UnicodeBig
    sun.java.launcher = SUN_STANDARD
    sun.jnu.encoding = UTF-8
    sun.management.compiler = HotSpot 64-Bit Tiered Compilers
    user.country = US
    user.language = en

Locale settings:
    default locale = English (United States)
    default display locale = English (United States)
    default format locale = English (United States)
    tzdata version = 2023c
 

A DESCRIPTION OF THE PROBLEM :
If I invoke ZonedDateTime.now() twice sequentially, the second invocation sometimes returns time before the first one. See test case below.

I've observed this behavior only on Apple hardware with Apple M CPU. It does not happen on other operating systems (Windows, Linux) with Intel x64 CPUs.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run ZonedDateTime.now() twice in a sequence and compare the returned values. See test case below for more details.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Second invocation of ZonedDateTime.now() returns value, that is the same as the first one or after the first one.
ACTUAL -
In some cases, second invocation returns value before the first one.

You need to repeat the sequence multiple times before it happens. On my machine, it usually happens within a couple of milliseconds.

---------- BEGIN SOURCE ----------
package bug.java.now;

import java.time.ZonedDateTime;
import org.junit.jupiter.api.Test;

class NowTest {

  @Test
  void test() {
    while (true) {
      var t1 = ZonedDateTime.now();
      var t2 = ZonedDateTime.now();
      if (t2.isBefore(t1)) {
        throw new RuntimeException("Is before " + t2 + " " + t1);
      }
    }
  }
}
---------- END SOURCE ----------

FREQUENCY : always