Tableau can handle all kind of dates and dateformats very well – but one option is lacking: the use of ISO8601 weeknumbers, the one which is used all over the world, especially in Europe. 

This formula will generate the right ISO weeknumber for you:

INT(datediff('day', #1900-1-01#, [Date])/7)*7 
+ 3,#1900-1-01#))) + 6)/7)

Tableau Desktop interprets 12/31/2017 as week 53 of 2017 instead of week 1 for 2018 (per the ISO 8601 standard) – therefore you need to ‘compensate’ the year also:

If datepart('week',[Date])=53 
THEN datepart('year',[Date])+1 
ELSE datepart('year',[Date]) end

(if you want to have this resolved in Tableau itself: please vote for this must have feature on UPDATE: this feature has been implemented in Tableau!

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