The time and date format for the Pantheon Greeter is defined in its translation files here: https://github.com/elementary/greeter/tree/master/po
You can fork the project, make the change to po file and open a pull request so that the developer can review your change and publish it with the next version. There is no coding involved. Open the po file for your language and find
#. / Time display section. There you'll find a link to the guidelines on how to set
msgstr in that section to match your country's setup. For the reference, you can also open another po file and look there.
It's not possible to change that since it depends on how it was translated. There's a bug reported already about it.