Day of week is compatible with old versions,
ie, 0 is sunday, 6 is saturday, so it shouldn't be
used to calculate the day of the week with the given
week number. Year day is also backwards compatible,
ie, one (1) less then from the year_day() function.
If this function is called in a Week object that begins with
the first week of a year, it returns the previous year if that
is where the week starts. To keep the representation
unambiguous, the returned week number is then one more than
the number of weeks in that year.
E.g. Week(2008,1)->datetime() will return year 2007 and week
53 since the first week of 2008 starts in 2007.