front end date field format doesn't work? fields[dateField][date]
I suspect it might be your values. Try testing with this hard coded in and see if it works:

&lt;input type="hidden" id="endDate" name="fields[endDate][date]" value="12/30/2017" /&gt;
&lt;input type="hidden" id="endTime" name="fields[endDate][time]" value="2:15 AM" /&gt;


I always reference what's on this StraightUpCraft guide, and I try to be as exact as possible to the examples. You can also just inspect date fields inside a saved entry in the CMS to see what the values look like and mimic that on the front end.