Hi,
sorry for late reply, it was night for us.

LastVisit date uses a language constant to set output format. the language constant should be DATE_FORMAT_LC1.

You can use Joomla language override feature to change this:
- Go to "Extensions->Language->Overrides
- Click to "New" button
- Set form like screenshot, default value (for english language) is "l, d F Y", you can change value in according with PHP date function (http://php.net/manual/en/function.date.php) so you can put something like "l, d F Y H:i"