In the documentation for date formats, epoch_millis has a maximum char length of 13 (year range of 1653 and 2286).
Also true for epoch_seconds.
Elasticsearch uses DateTime(millis, DateTimeZone.UTC) to construct an immutable Joda DateTime, where millis is a Java Long value. millis can certainly accept more than 13 chars of digits.
I'd like to understand why this limit exists when dealing with epochs.
Cross-post alert:
I've opened a GitHub issue and have a PR available, pending the outcome of this discussion.