Has this problem been solved?
I sort of “hacked” the general-template.php by adding my timezone.
I am in Rio (UTC-0300 or -0200 during the summer). I changed line 1008 to read:
$unixmonth = mktime(4, 0 , 0, $thismonth, 1, $thisyear);
Where I add <4> hours to the time. This seems to have “solved” it.