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.

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