Zero date in timestamp

[zerocrates]

See http://omeka.org/forums/topic/record-relations-plugin-installation-issue

Newer MySQL versions by default don't allow the "all-zero" timestamps they used to. You could replace that default with CURRENT_TIMESTAMP (as the zero date was usually just a workaround for wanting two timestamps on one record, which was previously illegal), or with some arbitrary but valid date. The arbitrary date method then requires you to use something like our Timestamp mixin to correctly set the date from the PHP side.

On a quasi-related note, I'd guess that the current timestamp doesn't update correctly. Usually ON UPDATE clauses don't really work with the Omeka ORM (as it always specifies a value for every column).

See also omeka/Omeka#678.