LibreOffice pivot table group by month

Im having data like this:

Date      Customer   Amount Month
09-01-12  A          20     =A2 (formatted so it shows month name)
10-01-12  B          10     =A3 (formatted so it shows month name)
18-01-12  B          25     etc
05-02-12  A          15     etc

I want to use a pivot table to see the total amount per month per customer. e.g.

Customer   Jan  Feb  Total
A          20   15   35
B          35        35
Total      55   15   70

I added an extra column to the data, with the month formatted as 'Jan', 'Feb' etc. In the pivot table it gives a column for each date, it does not group by month. So the result is this:

Customer   Jan  Jan  Jan Feb  Total
A          20            15    35
B               10   25        35
Total      20   10   25   15   70

How do I fix this?

I think the problem is that the "Month" column has different values for each date. So LibreOffice doesn't understand you want to group the output by distinct months.

Thus, a solution could be to "construct" a date based on cell A2, ignoring the day:

• Replace the Formula "=A2" in the "Month" column by

  =DATE(YEAR(A2),MONTH(A2),"1")

  This way, you will have the same date for every month cell, depending on the original date.

• Then, format that column using the pattern "MMMM" to display the date as "January".

• Now, refresh the Pivot Table or re-create it with "Month" in "Column Fields", "Customers" in "Row Fields" and "Sum - Amount" in "Data Fields".

This should yield the desired result.

I'm not sure whether this was possible in LibreOffice that was current in the time when this question was asked but now this is quite easy to do, so if anyone stumbles upon this question this might help:

Using data from the input table (btw. for this last column "Month" is redundant) one should do the following.

Create Pivot table:

1. Select the table and from the menu select Data > Pivot Table > Create...
2. In the Column Fields is the Date field
3. In Row Fields is the Customer field
4. In Data Fields the Amount field

When the Pivot table is created:

1. Select/click on any cell in the row which contains dates
2. From the menu select Data > Group and Outline > Group
3. Select Group By > Intervals
4. Check Months (or any interval you would like)