I have noticed that contrary to most pages that have release tables (release group page, label page, artists' releases page and maybe others).
There may be several sort criterias at the same time, but the release event sort order seems inconsistent (broken, wrong) in those two page types :

• release series where undated releases come first instead of last
• release collection where (currently on page 13) we can see some dates in the middle of undated releases. It would be because (only in collections AFAIK), releases having (at least) one release event without date appear last (cf. IRC by chirlu`)

Those two should use the general (good) rule, IMO.

How to determine a release date : take the earliest of that rule :

• precise dates first
• unprecise dates after
• undated last

How to determine a release group date : sort its releases with same rule then determine the earliest.

How to sort releases or release groups by date (when we want to sort them by date) : same rule.

Good examples :

1. 1976
2. 1977-05
3. 1977-06-24
4. 1977-06
5. 1977
6. no date set