Следующие строки в русском переводе gdm.po:

#. translators: This is the time format to use when both
#. * the date and time with seconds are being shown together.
#.
#: ../gui/simple-greeter/gdm-clock-widget.c:70
msgid "%a %b %e, %l:%M:%S %p"
msgstr "%a %b %e, %l:%M:%S %p"

#. translators: This is the time format to use when both
#. * the date and time without seconds are being shown together.
#.
#: ../gui/simple-greeter/gdm-clock-widget.c:76
msgid "%a %b %e, %l:%M %p"
msgstr "%a %b %e, %l:%M %p"

#. translators: This is the time format to use when there is
#. * no date, just weekday and time with seconds.
#.
#: ../gui/simple-greeter/gdm-clock-widget.c:83
msgid "%a %l:%M:%S %p"
msgstr "%a %l:%M:%S %p"

#. translators: This is the time format to use when there is
#. * no date, just weekday and time without seconds.
#.
#: ../gui/simple-greeter/gdm-clock-widget.c:92
msgid "%a %l:%M %p"
msgstr "%a %l:%M %p"

не переведены, что приводит к отображению времени в gdm в 12-часовом формате (см. также https://bugs.launchpad.net/ubuntu/+source/gdm/+bug/435788).
В данных строках %l:%M %p необходимо заменить на %H:%M