Line 59 of verifyemail.php should be changed to this
$olddate = mktime(0,0,0,intval(date("%m")), date("%d") - $days, intval(date("%y")));
because the date function returns a string value even if the result is a number.
Unless you perform a calculation with other numbers the value is kept as a string (parameters 4 and 6)
Parameter 5 performs a calculation and therefore is converted to a number.