The calculation is correct only.
IF [Date] <= TODAY() AND DATEDIFF('month',[Date],TODAY())=0 THEN [Sales] END
What exactly the issue you are facing?
My observation: I've data in excel like below:
Date Sales 15-Apr 1 16-Apr 2 17-Apr 3 18-Apr 4 19-Apr 5 20-Apr 6 21-Apr 7 22-Apr 8 23-Apr 9 24-Apr 10
When double click on the Date cell in excel, it will show the year as 2018. When I copy the same to the clipboard and paste in Tableau, Tableau considered year as 1900.
Hope you are also facing the same?
Remove the ' Else 1' clause in your formula or you can use the formula below.
Create calculated field [MTD Sales]
IF DATETRUNC('month', [ActualDate]) = DATETRUNC('month', TODAY()) AND [ActualDate] <= TODAY() THEN [Sales] END
Hope this helps.