I tried that and it worked!
The issue gets a little more complicated if you use Blesta for clients in different countries, because in the US the format is m/d/y and in others (most of latin america, and some european even) the format would be d/m/y, and the invoice gets a little confusing.
It would be nice that you can use the month's name translated according to the language your client is using, so using the default formatting would work for most (if not all) of the cases.
I found these, apparently a locale can be set for dates, days of the week, and so on.
https://www.php.net/manual/en/function.strftime.php
https://www.php.net/manual/en/function.setlocale.php
Please, consider the possibility of adding this behavior to Blesta.
Thanks.