How can I set gdm login screen time to 24 hour format?

How can I set gdm login screen time to 24 hour format? References I installed CentOS 6.4 within VMWare 7 by easy setup. Afterwards I had to change locale settings to German. Everythings works fine accept that the clock on the login screen is still in 12-hours-AM/PM-format. I want to change it to the 24-hour-format. GDM Version is 2.30.4. I tried the suggestions from the answers to a similiar question and another question. However, the suggested command gconf-editor seems not to provide a way to configure the clock format at the login screen. The other command gsettings is not available in my environment. How can I obtain it? There are other suggestions involving gdm.conf but the only gdm.conf in my environment is a XML file and not located in the directories specified in these suggestions. At Red Hat there seems to be a support article about this issue, however bp;dr. How can I set the login screen time to the 24 hour format?