Test der Norm-week/year Funktion (ISO 8601)

 
  from Wikipdia ISO 8601:
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  There are mutually equivalent descriptions of week 01:
    * the week with the year´s first Thursday in it (the formal ISO definition),
    * the week with 4 January in it,
    * the first week with the majority (four or more) of its days in the starting year, and
    * the week starting with the Monday in the period 29 December - 4 January.

  If 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in week 01. 
  If 1 January is on a Friday, Saturday or Sunday,
  it is in week 52 or 53 of the previous year. 
  28 December is always in the last week of its year.
  The week number can be described by counting the Thursdays: week 12 contains the 12th Thursday of the year.

  The ISO week numbering year starts at the first day (Monday) of week 01 and ends at the Sunday before the new ISO year 
  (hence without overlap or gap). It consists of 52 or 53 full weeks. The ISO week numbering year number deviates from the 
  number of the calendar year (Gregorian year) on a Friday, Saturday, and Sunday, or a Saturday and Sunday, or just a Sunday, 
  at the start of the calendar year (which are at the end of the previous ISO week numbering year) and a Monday, 
  Tuesday and Wednesday, or a Monday and Tuesday, or just a Monday, at the end of the calendar year 
  (which are in week 01 of the next ISO week numbering year). For Thursdays, the 
  ISO week-numbering year number is always equal to the calendar year number.