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In 2.3.x, date format settings are found in /language/yourlanguage/global.php on lines 167 - 169.
I've altered mine to be:
define("_DATESTRING","d/m/Y H:i:s");
define("_MEDIUMDATESTRING","d/m/Y H:i");
define("_SHORTDATESTRING","d/m/Y");
Also checkout lines 28 - 31 of /language/yourlanguage/locale.php
Mine reads:
define("_TODAY", "\T\o\d\a\y H:i");
define("_YESTERDAY", "\Y\e\s\\t\e\\r\d\a\y H:i");
define("_MONTHDAY", "d/m/Y H:i");
define("_YEARMONTHDAY", "d/m/Y H:i");
HTH