I have had this issue. The date format to search by is 2016-03-03 21:20:59Z This is inconvenient, so as a work around you can create a formula field like this to search on. It will give you the date in M/D/YYYY format: text(Month(Datevalue(Sent_At__c)))&“/”& text(Day(Datevalue(Sent_At__c)))&“/”&text(Year(Datevalue(Sent_At__c)))

Adding to Raymonds formula.  If you want, you can add this formula field to the search fields,  and not show it in the table.  Then when users search they will get results,  but you can still have the editable date time field shown in your table.