Comment 1 for bug 515042

I don't think it's possible to follow the user's locale setting, because there is no unambiguous 1:1 mapping between locales and date formats. For example, read the first paragraph under the subheading "Date" at http://en.wikipedia.org/wiki/Date_and_time_notation_by_country#Canada

Another reference is the gedit "Date" plugin, code for which is here:
http://git.gnome.org/browse/gedit/tree/plugins/time/gedit-time-plugin.c
...they seem to provide a number of options in a drop-down, and default to ISO 8601.