Currently the german translation of gdm uses the english date/time formats instead of the localized ones. 

Below the relevant parts of de.po with proposed fixes.

#: ../gui/simple-greeter/gdm-clock-widget.c:66
msgid "%l:%M:%S %p"
[Current translation: 12 hour format]
msgstr "%l:%M:%S %p"

[Should be: 24 hour format]
msgstr "%H:%M:%S"

#: ../gui/simple-greeter/gdm-clock-widget.c:66
msgid "%l:%M %p"
[Current translation: 12 hour format]
msgstr "%l:%M %p"

[Should be: 24 hour format]
msgstr "%H:%M"

#: ../gui/simple-greeter/gdm-clock-widget.c:72
msgid "%a %b %e"
[Current translation: formally not correct in germany]
msgstr "%a %b %e"

[Should be: (this is like clock in gnome-panel does it)]
msgstr "%a, %d. %b"