I think are you looking for:
// use this format for short date display
$rcmail_config['date_short'] = 'D H:i';
// use this format for detailed date/time formatting
$rcmail_config['date_long'] = 'd.m.Y H:i';
// use this format for today's date display
$rcmail_config['date_today'] = 'H:i';
in config/main.inc.php. You can set the format to what ever you like using PHP date formatting.
