This would help alot if there are many files (e.g. pictures) which all have the same date, but different timestamps. Probably it does not make much sense to have "acces-time" / "alternation-time" as separate column, since to sort by "time" usually is not what is needed. So I would propose to add a dropdown "date/time format" in the listcolumn-settings: date + time date only time only A more sophisticated fix could be to have the possibility to specify a representation format from an input field: JJJJ.MM.DD hh:mm:ss DD.MM.JJJJ ... but IMO this would be overkill.
Looks like this was already fixed in 2007, however I did not find any related setting while using the the master branch.
https://bugzilla.xfce.org/show_bug.cgi?id=3265
*** This bug has been marked as a duplicate of bug 14527 ***
Actually I just failed to see that Preferences --> Display --> Date --> Format as well changes the date-style for detailed view O_o