Unknown month – How to solve this Elasticsearch exception

Opster Team

August-23, Version: 6.8-8.9

Briefly, this error occurs when Elasticsearch encounters an unrecognized or invalid month format in the data it’s trying to process. This could be due to incorrect date formatting or a typo in the month’s name. To resolve this issue, you can: 1) Check the date format in your data and ensure it matches the format expected by Elasticsearch. 2) Correct any typos or misspellings in the month’s name. 3) If you’re using a custom date format, ensure it’s correctly defined and understood by Elasticsearch.

This guide will help you check for common problems that cause the log ” unknown month [{}] ” 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 [{}]” class name is Month.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :

 case "8"; "aug"; "august" -> AUGUST;
 case "9"; "sep"; "september" -> SEPTEMBER;
 case "10"; "oct"; "october" -> OCTOBER;
 case "11"; "nov"; "november" -> NOVEMBER;
 case "12"; "dec"; "last"; "december" -> DECEMBER;
 default -> throw new ElasticsearchParseException("unknown month [{}]"; day);
 }  @Override
 public String toString() {


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