This has been reported downstream at:
http://bugs.gentoo.org/show_bug.cgi?id=342557

If /usr/share/zoneinfo/Factory is set as /etc/localtime produces an error when
calling date that the local timezone must be set:

  /GDateTime/new_full:                                                 OK
  /GDateTime/now:                                                      OK
  /GDateTime/printf:                                                   **
ERROR:gdatetime.c:818:test_GDateTime_printf: assertion failed (__p == (dst)): ("Local time zone must be set--see zic manual page" == "")
FAIL
GTester: last random seed: R02S1f1f9903edbed26b5152d1ba78e0dd5a
/bin/sh: line 1: 20293 Terminated              MALLOC_CHECK_=2 MALLOC_PERTURB_=$((${RANDOM:-256} % 256)) ../../glib/gtester --verbose testing option-context keyfile fileutils printf protocol rand strfuncs string markup-parse markup-collect markup-escape markup-subparser array-test hostutils gvariant mem-overflow utils regex base64 sequence scannerapi shell collate utf8-pointer utf8-validate utf8-misc checksum hash date node convert list slist queue tree uri dir pattern logging error bookmarkfile gdatetime
make[4]: *** [test-nonrecursive] Error 143
make[4]: Leaving directory `/var/tmp/portage/dev-libs/glib-2.26.0-r1/work/glib-2.26.0/glib/tests'
make[3]: *** [check-am] Error 2
make[3]: Leaving directory `/var/tmp/portage/dev-libs/glib-2.26.0-r1/work/glib-2.26.0/glib/tests'
make[2]: *** [check-recursive] Error 1
make[2]: Leaving directory `/var/tmp/portage/dev-libs/glib-2.26.0-r1/work/glib-2.26.0/glib'
make[1]: *** [check] Error 2
make[1]: Leaving directory `/var/tmp/portage/dev-libs/glib-2.26.0-r1/work/glib-2.26.0/glib'
make: *** [check-recursive] Error 1

Feel free to close this as WONTFIX if you want, we simply report this to let you know the problem and know if this is expected or test should be more solid instead.

Thanks