how to print login_days with user_stats module?
As Berdir stated in the comment, user_stats_get_stats('login_days'...) returns a number of days so you can't use this with format_date, also I tried this on my site and if you're the author of the node this function will return nothing, but if you try with other users that have logged in, this should display the number of days since their last access.

In fact if you're only using this module for this function, it may be a little overkill and you should better do this manually:

$u = user_load(31);
print floor((time() - $u-&gt;access) / 86400); 


Edit: 12/06 11:57
If you want to format the last access date then do:

$u = user_load(31);
print format_date($u-&gt;access, 'custom', 'j M H:i');
//will display 12 Jun 12:03 with hour formated in 24H.