DATETIME columns don't contain TZ information and as such don't represent an exact point on the timeline but rather a logical date and time, such as "2018-01-13 09:48:27". Hence it's questionable to represent such columns as epoch millis, as this requires a time zone for mapping the logical date to such exact point on the time line.

Currently, UTC is used for this, but this is a rather arbitrary decision and not really transparent to downstream consumers. Therefore, a new mapping mode should be provided, which exports DATETIME columns as strings, keeping their nature of being logical dates and not exact points on the timeline.