I too just finished struggling with this problem except I needed all with no date where you need the opposite.
I found that this works but don't know if there is a better way:
'where' => "ship_date != '0000-00-00 00:00:00'", // filters out all with no date.
since date fields are stored padded with zeros and is not as a NULL
Ron Conring
Conring Automation Services
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