If I specify a value for elasticSearch.date.formats, I do not believe that this value is honored when unmarshalling.
See: https://github.com/puneetbehl/elasticsearch-grails-plugin/blob/03931f05aa1d337603f5929b3f78924c8680af76/src/main/groovy/grails/plugins/elasticsearch/conversion/unmarshall/DomainClassUnmarshaller.groovy#L253
I would imagine that this line should first check the config for a value before defaulting to this hardcoded format.