Function g_date_time_format() is explained in a reference manual as following:

========
In contrast toglib-2.31.10/glib/gi18n.h strftime(), this function always produces a UTF-8 string, regardless of the current locale. Note that the rendering of many formats is locale-dependent and may not match the strftime() output exactly. 
========
(http://developer.gnome.org/glib/stable/glib-GDateTime.html#g-date-time-format)

But g_date_time_format() produces non-UTF-8 string in some locales actualy.
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cat > reproduce_bug.c <<EOF
#include <stdio.h>
#include <locale.h>
#include <glib/gdatetime.h>

int main( int argc, char** argv ){
  char *result;
  GDateTime *datetime;
  GTimeVal tv;
  setlocale( LC_ALL, "" );
  tv.tv_sec = tv.tv_usec = 0;
  datetime = g_date_time_new_from_timeval_local (&tv);
  result = g_date_time_format (datetime, "%A %a %B %b");
  printf ( "date: %s\n", result );
  return 0;
}
EOF
gcc reproduce_bug.c $( pkg-config --cflags --libs glib-2.0 )

env LC_TIME=C           ./a.out
env LC_TIME=ja_JP.eucjp ./a.out
========

Reason, g_date_time_format() calls nl_langinfo() or g_dpgettext() and uses these return values directly.
Both functions return locale-depend strings.

This bug causes crash of gnome-shell in some locales.