Unknown month number – How to solve this Elasticsearch exception

Opster Team

August-23, Version: 6.8-8.9

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Briefly, this error occurs when Elasticsearch encounters an invalid month number in a date field. Elasticsearch expects month numbers to be between 1 and 12, and any value outside this range will trigger this error. To resolve this issue, you can either correct the invalid month number in your data or implement error handling in your application to prevent invalid dates from being sent to Elasticsearch. Additionally, you can use a script to validate date fields before indexing documents.

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This guide will help you check for common problems that cause the log ” unknown month number [{}] ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: plugin.

Log Context

Log “unknown month number [{}]” class name is Month.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :

 case 8 -> AUGUST;
 case 9 -> SEPTEMBER;
 case 10 -> OCTOBER;
 case 11 -> NOVEMBER;
 case 12 -> DECEMBER;
 default -> throw new ElasticsearchParseException("unknown month number [{}]"; month);
 }  public static Month resolve(String day) {
 return switch (day.toLowerCase(Locale.ROOT)) {


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