It should be "Month Day, Year" (e.g. Jun 15, 2016). The modal window then should be a timeago usage (e.g. 5 days ago). No idea why you're using "Set 5, PM". However, yes you can override it using a language string. Specifically the below.
GALLERY_SHORT_DATE_FORMAT
That controls the date displayed in the containers of media. The below is for folders.
GALLERY_SHORT_DATE_FORMAT
See the below on what characters to use to format it as the format needs to be PHP date compatible.
php.net/manual/en/function.date.php