Alex Marquardt explains how to convert local timestamps to ISO 8601 format in Elasticsearch using an ingest pipeline, which is crucial when a timestamp field is submitted without timezone information and assumed to be in UTC. By using an ingest processor, timestamps can be accurately converted if the local timezone is known, preventing display issues in applications like Kibana. The example provided demonstrates how a date processor can transform a timestamp from the Europe/Madrid timezone into the ISO 8601 format, considering daylight savings time changes. Marquardt shows how to simulate the process and verify that the pipeline correctly handles timezone offsets, including adjustments for daylight savings. Additionally, a practical example of inserting and retrieving a document within Elasticsearch is provided, emphasizing the importance of having unambiguous timestamps for accurate data representation. The blog encourages users to try this setup in their Elasticsearch cluster or test it using the free trial of Elasticsearch Service, highlighting the community forums as a resource for further questions on the topic.