The tag originaldate should be set from the the earliest release date in the release group. However if a release date has a shortened format like YYYY or YYYY-MM, it supersedes other more precise release dates, like YYYY-MM or YYYY-MM-DD respectively, that share the same release year or month values.

ex:
http://musicbrainz.org/release-group/5d6e21e1-deb5-428e-bb42-c2a567f3619b
has a release date with 2010-11 and also 2010-11-22, but it is the 2010-11 that is chosen

ex:
http://musicbrainz.org/release-group/20aaac17-e4e8-373c-99cc-211321b82832
has a release date with 1989 and 1989-09-15, but it is the 1989 that is chosen