Excel pivottable calculate value of previous month

I have the following table (ps. here is the example created in office 2016: https://www.dropbox.com/s/tu7b9u1z7pi5i3d/Book1.xlsx)

date amount
2016-01-05  2
2016-01-08  5
2016-02-09  3
2016-02-15  1
2016-03-18  5
2016-03-19  6

Now I created a Pivot table with date(month) as a filter and amount as value. This gives me the following table

date (Month)    mrt
Sum of amount   prevMonth
11

Now I want to calculate the same value for the previous month which was selected. In this case that would be february and the value underneath prevMonth would be 4.

Note, that I do not actually want to select both March and February in the filter.

How do I do this? I currently have =CALCULATE([Sum of amount]]),PREVIOUSMONTH('Table1'[date])), however this does not seem to do the trick.