Internally, when you map any date as a date type, it will be converted to an epoch+milliseconds timestamp. Internally. This is how Elasticsearch is able to perform range queries against dates. But in the _source, your date will remain in the string representation you submitted.
See https://www.elastic.co/guide/en/elasticsearch/reference/2.3/mapping-date-format.html
Because it's stored in epoch, and epoch is presumed to be in UTC, then all dates should be submitted in UTC, or ISO8601 with a timezone offset.