The defaults for formating of the date and time strings are configured in the TS-setup of tt_news.
Notice: if you have one of the ts_language_xx extensions installed and configured as default language, these extension will override the default settings for the tt_news time formating.The language specific TS settings for several extensions can be found in the file "ext_typoscript_setup.txt" in the (ts_language-) extensions install directory (e.g. "/typo3/ext/ts_language_de/ext_typoscript_setup.txt" for the german settings).