+ IsValidDate() checks only day and month regarding the year, not Gregorian
  cut-off date as now does IsValidAndGregorian().
+ Normalize() carries over invalid day and month values to next months and
  years.
* All methods that return or internally use a day count now internally
  normalize the date values, without modifying the actual Date instance. So,
  if the date is not valid you may get unexpected results.
  * Previously, a date with month>12 would had accessed the days-of-month
    array out of bounds on all such methods. So you would had gotten
    unexpected results anyway..
  * Affected methods are:
    GetDayOfYear()
    GetWeekOfYear()
    GetDaysInMonth()
    static DateToDays()