Option 2 (MTD)

1. Select Analysis > Create Calculated Field 
2. Name the field MTD Sales, enter the following calculation, then click OK: 
   [Order Date] <= TODAY() AND
DATETRUNC( "month", [Order Date]) = DATETRUNC("month", TODAY() )
3. Drag MTD onto the filter shelf and select "True"

Option 3 (Dynamic MTD/YTD)

1. Create a parameter and name it "YTD/MTD" - String
2. Create a parameter and name it "Anchor Date" - Date
3. Create a parameter and name it "Offset" - Int
4. Select Analysis > Create Calculated Field 
5. Name the field Dynamic Sales, enter the following calculation, then click OK: 
   [Order Date] <= DATEADD( [Date Part],[Offset], [Anchor Date] ) AND
DATETRUNC( [Date Part], [Order Date]) = DATETRUNC([Date Part], DATEADD([Date Part], [Offset], [Anchor Date] ))
6. Drag MTD onto the filter shelf and select "True"