Hi,
last visit date format use Joomla language string, so output is different for each language.

Language Constant is DATE_FORMAT_LC1 and english value is "l, d F Y"

l => day (text)
d => day (number)
F => month (text)
Y => Year (4 digits)
read here for more infos: http://php.net/manual/it/function.date.php

You can override this language string with your value, go into Administration->Extensions->Language Manager->Overrides(left menu), now you can create a override for each language

some examples:

Constant: DATE_FORMAT_LC1
Text: F j, Y, g:i a
Result: March 10, 2001, 5:16 pm

Constant: DATE_FORMAT_LC1
Text: m.d.y
Result: 03.10.01

Why language strings? because for each language the format is different, and goes according to the habits and standards of that country.