I think I’ve found the root cause of why some date pickers are behaving oddly.
<c:if test="${criteria.startDate eq date and criteria.startDate eq null}">
criteria.startDate is a date object, however date is a calendar object representing the current date and should never be null. Hence, it’s impossible for the above test to return true.
The code is total garbage and I wrote it. 
I wish I had an excuse – I was still learning JSP, we were in a rush, it didn’t crash anything, but frankly I should have spotted the dumbness of it before now. Oh well, on with the fixing.
Very True Mood: 
aggravated
aggravated