+ 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()
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