You didn't mention the version of Tableau that you are working with so I have created the sample attached in version 10.
I managed the first situation slightly differently than the method mentioned.
If [Select State]="All" then "Show"
If [Select State]=[State] then "Show" else "Hide" End
and then I manage the Date issue in pretty much the same way:
If [Select Month]=datetrunc('month',[Order Date]) then "Show" else "Hide" End
You can add in the "All" by selecting a date to add to the parameter that is not in the data set. In the example I supplied, I choose 1/1/2000 and made this calc to filter with:
If [Select Month]=#1/1/2000# then "Show" elseif
[Select Month]=datetrunc('month',[Order Date]) then "Show"
else "Hide" End
I hope this helps.
Sample_Dates_Filtering.twbx 1.9 MB