ADDITIONAL SYSTEM INFORMATION :
OS name: "mac os x", version: "14.6.1", arch: "aarch64", family: "mac"
Java version: 24, vendor: Oracle Corporation,


A DESCRIPTION OF THE PROBLEM :
The problem is based on the usage of "Pattern Letters and Symbols" in relationship with DateTimeFormatter.ofPattern("E LLL d HH:mm:ss yyyy XX").format(datetime);
The usage of the three letter "LLL" is described https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/time/format/DateTimeFormatter.html#patterns using the symbol "L" three times means having a result of "Oct" (length of three) as an example for October etc.
Also a supplemental hint from the documentation: "Number/Text: If the count of pattern letters is 3 or greater, use the Text rules above. Otherwise use the Number rules above."
That means that for all months the resulting output should contain only three letters like "Oct", "Jan", "Feb" etc. but unfortunately the output for September results in "Sept" (four characters instead). The September seemed to be the only exception (see actual result). For all other months it results correctly in three letter results.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Use the following code:

var time = LocalDateTime.of(2023, 9, 12, 15, 47, 32, 200);
var datetime = ZonedDateTime.of(time, ZoneId.systemDefault());
String formatted = DateTimeFormatter.ofPattern("E LLL d HH:mm:ss yyyy XX").format(datetime);
System.out.println(formatted);

I can also change the usage for ZoneId to this:
var datetime = ZonedDateTime.of(time, ZoneId.of("Europe/Berlin"));
and issue still persists.
Also changing to:
var datetime = ZonedDateTime.of(time, ZoneId.of("America/Phoenix"));
does not change a thing. Issue stil persists.


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Sun Apr 13 00:08:27 2025 +0200
Tue Sep 12 15:47:32 2023 +0200
Thu Sep 12 15:47:32 2024 +0200
Tue Oct 17 22:49:38 2023 +0200
Tue Jan 17 22:49:38 2023 +0100
Fri Feb 17 22:49:38 2023 +0100
Fri Mar 17 22:49:38 2023 +0100
Mon Apr 17 22:49:38 2023 +0200
Wed May 17 22:49:38 2023 +0200
Sat Jun 17 22:49:38 2023 +0200
Mon Jul 17 22:49:38 2023 +0200
Thu Aug 17 22:49:38 2023 +0200
Sun Sep 17 22:49:38 2023 +0200
Tue Oct 17 22:49:38 2023 +0200
Fri Nov 17 22:49:38 2023 +0100
Sun Dec 17 22:49:38 2023 +0100
ACTUAL -
Sun Apr 13 00:08:27 2025 +0200
Tue Sept 12 15:47:32 2023 +0200
Thu Sept 12 15:47:32 2024 +0200
Tue Oct 17 22:49:38 2023 +0200
Tue Jan 17 22:49:38 2023 +0100
Fri Feb 17 22:49:38 2023 +0100
Fri Mar 17 22:49:38 2023 +0100
Mon Apr 17 22:49:38 2023 +0200
Wed May 17 22:49:38 2023 +0200
Sat Jun 17 22:49:38 2023 +0200
Mon Jul 17 22:49:38 2023 +0200
Thu Aug 17 22:49:38 2023 +0200
Sun Sept 17 22:49:38 2023 +0200
Tue Oct 17 22:49:38 2023 +0200
Fri Nov 17 22:49:38 2023 +0100
Sun Dec 17 22:49:38 2023 +0100

---------- BEGIN SOURCE ----------

import java.time.Instant;
import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;

public class FormattingIssue {

  private static final String FORMATTING = "E LLL d HH:mm:ss yyyy XX";

  static void formattingCurrentZonedDateTime() {
    var time = Instant.now();
    var datetime = ZonedDateTime.ofInstant(time, ZoneId.systemDefault());
    String formatted = DateTimeFormatter.ofPattern(FORMATTING).format(datetime);
    System.out.println(formatted);
  }

  static void formattingSeptember23ZonedDateTime() {
    var time = LocalDateTime.of(2023, 9, 12, 15, 47, 32, 200);
    var datetime = ZonedDateTime.of(time, ZoneId.systemDefault());
    String formatted = DateTimeFormatter.ofPattern(FORMATTING).format(datetime);
    System.out.println(formatted);
  }

  static void formattingSeptember24ZonedDateTime() {
    var time = LocalDateTime.of(2024, 9, 12, 15, 47, 32, 200);
    var datetime = ZonedDateTime.of(time, ZoneId.systemDefault());
    String formatted = DateTimeFormatter.ofPattern(FORMATTING).format(datetime);
    System.out.println(formatted);
  }

  static void formattingOctoberZonedDateTime() {
    var time = LocalDateTime.of(2023, 10, 17, 22, 49, 38, 251);
    var datetime = ZonedDateTime.of(time, ZoneId.systemDefault());
    String formatted = DateTimeFormatter.ofPattern(FORMATTING).format(datetime);
    System.out.println(formatted);
  }

  static void formattingMonthsZonedDateTime() {
    for (var month = 1; month <= 12; month++) {
      var time = LocalDateTime.of(2023, month, 17, 22, 49, 38, 251);
      var datetime = ZonedDateTime.of(time, ZoneId.systemDefault());
      String formatted = DateTimeFormatter.ofPattern(FORMATTING).format(datetime);
      System.out.println(formatted);
    }
  }

  public static void main(String[] args) {
    formattingCurrentZonedDateTime();
    formattingSeptember23ZonedDateTime();
    formattingSeptember24ZonedDateTime();
    formattingOctoberZonedDateTime();
    formattingMonthsZonedDateTime();
  }
}

---------- END SOURCE ----------