The link shared by Norbert should definitely help. You can tweak the calculation described there just a bit to get the number of business days before today:
DATEDIFF("weekday", [Start Date], [Today])
- 2 * (DATEPART('week', [Today]) -DATEPART('week', [Start Date]))
+ (IF DATENAME('weekday',[Today]) = 'Saturday' OR DATENAME('weekday',[Start Date]) = 'Sunday'
THEN 0 ELSE 1 END)
[Today] is a parameter but can be replace with TODAY(). A similar approach can be taken for days after today.
The workbook attached has both calculations.
Hope this helps!
BusinessDays.twbx 86.6 KB
Further to the posts here already I would like to suggest;
1. Search the forum. The question has been asked hundreds of times before
2. Post your Tableau Packaged Workbook including your sample data along with some more details on what you consider to be 'business days'.
Based on our own environment and experience we assume that the everybody on this planet has "free" acces to the internet and a "semantic search" will give the same result.
For sure this is not always the case.