How to adapt the date format in the admin/content page?
Date (and time) formats can be configured via admin/config/regional/date-time. It can be used to specify the format for data types Long, Medium and Short.

Other tabs that are available on that same page:


Formats (to define your own custom formats).
Localize (for language specific tuning).
Date Views (for date views specific to "day", "week" or "month").