If you have a class with a "date" attribute which is marked as "not required" and publish a object with a empty date the object will not be indexed. The value from solr is "invalid date format".

Steps to reproduce

Tested with standard 4.2.0 and eZfind 2.1 and eZFind 2.2 head.

Edit a class and add an attribute of datatype "Date" (not date and time)
Publish a object without any info in date
Reindex

The error in solr (if you run it in the foregrund) will be "invalid date format"

Best Regards
Adam