How to Calculate Week Number for a Given Date?

How do I calculate the week number for a given date?

Question

Given a date, how do you calculate the week number for that date within that year?

For example, in 2008, January 1st to January 6th are in week 1, and January 7th to the 13th are in week 2, so if your date was January 10th 2008, your week number would be 2.

Answer

Be aware that while your definition of nth week of the year is tenable, it is also not 'the' standard one.

ISO 8601 defines a standard for the representation of dates, times, and time zones. It defines weeks that start on a Monday. It also says Week 1 of a year is the one which contains at least four days from the given year. Consequently, the 29th, 30th, and 31st of December 20xx could be in week 1 of 20xy (where xy = xx 1), and the 1st, 2nd, and 3rd of January 20xy could be in the last week of 20xx. Further, there could be a week 53.

Then comes the interesting testing phase - when do you get week 53? One answer is on Friday 1st January 2010, which is in 2009-W53 (as, indeed, is Sunday 3rd January 2010). Similarly, Saturday 1st January 2005 is in 2004-W53, but Sunday 1st January 2006 is in 2005-W52.

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