Add NamedInstantPattern support to DatePatternConverter and tests

log4j-core-test/src/test/java/org/apache/logging/log4j/core/pattern/DatePatternConverterTest.java

@@ -21,6 +21,7 @@
 
 import java.util.Calendar;
 import java.util.Date;
+import java.util.stream.Stream;
 import org.apache.logging.log4j.core.AbstractLogEvent;
 import org.apache.logging.log4j.core.LogEvent;
 import org.apache.logging.log4j.core.time.Instant;
@@ -161,4 +162,27 @@ public void testGetPatternReturnsNullForUnixMillis() {
     public void testNewInstanceAllowsNullParameter() {
         DatePatternConverter.newInstance(null); // no errors
     }
+
+    private static final String[] PATTERN_NAMES =
+            Stream.of(NamedInstantPattern.values()).map(Enum::name).toArray(String[]::new);
+
+    @Test
+    public void testPredefinedFormatWithoutTimezone() {
+        for (final String patternName : PATTERN_NAMES) {
+            assertEquals(
+                    DatePatternConverter.decodeNamedPattern(patternName),
+                    DatePatternConverter.newInstance(new String[] {patternName}).getPattern());
+        }
+    }
+
+    @Test
+    public void testPredefinedFormatWithTimezone() {
+        for (final String patternName : PATTERN_NAMES) {
+            // Pacific Standard Time = UTC-08:00
+            assertEquals(
+                    DatePatternConverter.decodeNamedPattern(patternName),
+                    DatePatternConverter.newInstance(new String[] {patternName, "PST"})
+                            .getPattern());
+        }
+    }
 }

log4j-core/src/main/java/org/apache/logging/log4j/core/config/DefaultConfiguration.java

@@ -36,7 +36,7 @@ public class DefaultConfiguration extends AbstractConfiguration {
     /**
      * The default Pattern used for the default Layout.
      */
-    public static final String DEFAULT_PATTERN = "%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n";
+    public static final String DEFAULT_PATTERN = "%d{ABSOLUTE_PERIOD} [%thread] %-5level %logger{36} - %msg%n";
 
     /**
      * Only for tests.

log4j-core/src/main/java/org/apache/logging/log4j/core/pattern/DatePatternConverter.java

@@ -47,8 +47,6 @@ public final class DatePatternConverter extends LogEventPatternConverter impleme
 
     private static final String CLASS_NAME = DatePatternConverter.class.getSimpleName();
 
-    private static final String DEFAULT_PATTERN = "yyyy-MM-dd HH:mm:ss.SSS";
-
     private final InstantFormatter formatter;
 
     private DatePatternConverter(@Nullable final String[] options) {
@@ -62,7 +60,7 @@ private static InstantFormatter createFormatter(@Nullable final String[] options
         } catch (final Exception error) {
             logOptionReadFailure(options, error, "failed for options: {}, falling back to the default instance");
         }
-        return InstantPatternFormatter.newBuilder().setPattern(DEFAULT_PATTERN).build();
+        return InstantPatternFormatter.newBuilder().setPattern(NamedInstantPattern.DEFAULT.getPattern()).build();
     }
 
     private static InstantFormatter createFormatterUnsafely(@Nullable final String[] options) {
@@ -90,7 +88,17 @@ private static InstantFormatter createFormatterUnsafely(@Nullable final String[]
     }
 
     private static String readPattern(@Nullable final String[] options) {
-        return options != null && options.length > 0 && options[0] != null ? options[0] : DEFAULT_PATTERN;
+        return options != null && options.length > 0 && options[0] != null
+                ? decodeNamedPattern(options[0])
+                : NamedInstantPattern.DEFAULT.getPattern();
+    }
+
+    static String decodeNamedPattern(final String pattern) {
+        try {
+            return NamedInstantPattern.valueOf(pattern).getPattern();
+        } catch (final IllegalArgumentException ignored) {
+            return pattern;
+        }
     }
 
     private static TimeZone readTimeZone(@Nullable final String[] options) {

log4j-core/src/main/java/org/apache/logging/log4j/core/pattern/NamedInstantPattern.java

@@ -0,0 +1,77 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to you under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.logging.log4j.core.pattern;
+
+import org.jspecify.annotations.NullMarked;
+
+/**
+ * Named date-time patterns supported by {@link DatePatternConverter}.
+ */
+@NullMarked
+public enum NamedInstantPattern {
+    ABSOLUTE("HH:mm:ss,SSS"),
+
+    ABSOLUTE_MICROS("HH:mm:ss,SSSSSS"),
+
+    ABSOLUTE_NANOS("HH:mm:ss,SSSSSSSSS"),
+
+    ABSOLUTE_PERIOD("HH:mm:ss.SSS"),
+
+    COMPACT("yyyyMMddHHmmssSSS"),
+
+    DATE("dd MMM yyyy HH:mm:ss,SSS"),
+
+    DATE_PERIOD("dd MMM yyyy HH:mm:ss.SSS"),
+
+    DEFAULT("yyyy-MM-dd HH:mm:ss,SSS"),
+
+    DEFAULT_MICROS("yyyy-MM-dd HH:mm:ss,SSSSSS"),
+
+    DEFAULT_NANOS("yyyy-MM-dd HH:mm:ss,SSSSSSSSS"),
+
+    DEFAULT_PERIOD("yyyy-MM-dd HH:mm:ss.SSS"),
+
+    ISO8601_BASIC("yyyyMMdd'T'HHmmss,SSS"),
+
+    ISO8601_BASIC_PERIOD("yyyyMMdd'T'HHmmss.SSS"),
+
+    ISO8601("yyyy-MM-dd'T'HH:mm:ss,SSS"),
+
+    ISO8601_OFFSET_DATE_TIME_HH("yyyy-MM-dd'T'HH:mm:ss,SSSx"),
+
+    ISO8601_OFFSET_DATE_TIME_HHMM("yyyy-MM-dd'T'HH:mm:ss,SSSxx"),
+
+    ISO8601_OFFSET_DATE_TIME_HHCMM("yyyy-MM-dd'T'HH:mm:ss,SSSxxx"),
+
+    ISO8601_PERIOD("yyyy-MM-dd'T'HH:mm:ss.SSS"),
+
+    ISO8601_PERIOD_MICROS("yyyy-MM-dd'T'HH:mm:ss.SSSSSS"),
+
+    US_MONTH_DAY_YEAR2_TIME("dd/MM/yy HH:mm:ss.SSS"),
+
+    US_MONTH_DAY_YEAR4_TIME("dd/MM/yyyy HH:mm:ss.SSS");
+
+    private final String pattern;
+
+    NamedInstantPattern(final String pattern) {
+        this.pattern = pattern;
+    }
+
+    public String getPattern() {
+        return pattern;
+    }
+}

src/changelog/.3.x.x/4128_restore_named_instant_patterns.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<entry xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+       xmlns="https://logging.apache.org/xml/ns"
+       xsi:schemaLocation="https://logging.apache.org/xml/ns https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
+       type="changed">
+  <issue id="4128" link="https://github.com/apache/logging-log4j2/pull/4128"/>
+  <description format="asciidoc">Restore named date &amp; time patterns in Pattern Layout and export `org.apache.logging.log4j.core.pattern.NamedInstantPattern` for programmatic access</description>
+</entry>