DatePatternConverter incorrectly converting date pattern since Log4J 2.25.0

[tanmaybaid]

Description

DatePatternConverter incorrectly converting date pattern with starting Log4J 2.25.0 for following usecases:\

Usecase 1: Date pattern contains , (comma followed by a whitespace)

Example: %date{EEE, dd MMM yyyy HH:mm:ss zzz}{GMT}
Actual: Thu,' 18 Sep 2025 00:42:14 GMT
Expected: Thu, 18 Sep 2025 00:42:14 GMT

Usecase 2: Date pattern contains , (whitespace followed by a comma)

Example: %date{EEE ,dd MMM yyyy HH:mm:ss zzz}{GMT}
Actual: Thu ',18 Sep 2025 00:42:14 GMT
Expected: Thu, 18 Sep 2025 00:42:14 GMT

Usecase 3: Date pattern contains (two consecutive whitespaces)

Example: %date{EEE dd MMM yyyy HH:mm:ss zzz}{GMT}
Actual: Thu ' 18 Sep 2025 00:42:14 GMT
Expected: Thu 18 Sep 2025 00:42:14 GMT

Root Cause:

The issue is related to the "Date & Time Formatting" changes introduced in Log4J 2.25.0 as described here:
https://logging.apache.org/log4j/2.x/release-notes.html#release-notes-2-25-0-instant-format

Mitigation:

This issue is resolved by setting the log4j2.instantFormatter property to legacy

Configuration

Pattern layout with date pattern that includes , (comma followed by a whitespace) or , (whitespace followed by a comma) or (two consecutive whitespaces) - see usecases above with examples.

Version: 2.25.0

Operating system: MacOS and Linux

JDK: amazon-corretto-21.jdk and amazon-corretto-17.jdk

Logs

N/A

Reproduction

Pull request #3931 adds tests that fail due to this bug.

[ppkarwasz]

Hi @tanmaybaid, great catch! 🙌

You’ve run into a bug where multi-character unquoted literals (e.g., ", ", " ,", or two spaces) are merged incorrectly by the new instant formatter, which can inject a stray apostrophe at the boundary (e.g., Thu,' 18 …). I’ve opened #3932 to fix the parsing/merging logic.

Recommended workaround

Quote the literal runs in your pattern. This follows the JDK guidance to quote all literal text to avoid surprises:

“it is recommended to use single quotes around all characters that you want to output directly” — DateTimeFormatter Javadoc

For your examples:

• %date{EEE, dd MMM yyyy HH:mm:ss zzz} → %date{EEE', 'dd MMM yyyy HH:mm:ss zzz}
• %date{EEE ,dd MMM yyyy HH:mm:ss zzz} → %date{EEE' ,'dd MMM yyyy HH:mm:ss zzz}
• %date{EEE dd MMM yyyy HH:mm:ss zzz} → %date{EEE' 'dd MMM yyyy HH:mm:ss zzz}

Note

We don’t recommend switching to the legacy formatter. Quoting the literals is the safe, forward-compatible workaround, and the underlying bug is being addressed in #3932. For custom patterns like yours, the new formatter is much faster as it caches most of the data between calls.

Thanks again for reporting this!