Date and Time - Week Number definitions


I do know four different definitions of week numbering.

The first three definitions are in relation to the classic week and start about the begin of a year.

Windows-API: 0
  • Week containing --01-01 is the first week of that year. (Windows-API)
Windows-API: 1
  • First full week following --01-01 is the first week of that year. (Windows-API)
  • Week containing --01-07 is the first week of that year.
Windows-API: 2
  • First week containing at least four days is the first week of that year. (Windows-API)
  • The first calender week is the week, which contain at least four of the first seven january days. (DIN 1355)
  • Calendar week number: the ordinal number of the week, applying the rule that the first calendar week of a year is the one that includes the first Thursday of that year. (ISO 8601, Monday is defined as first weekday).
  • Week containing --01-04 is the first week of that year. (Gregorian Calendar)

The fourth definition is used in the United Kingdom. Dr. J. R. Stockton have described this definition on his Homepage.