I would

1. Create a new attribute with the date converted to a month/year format only, e.g. 1/2018.
2. Ensure that your data is sorted in date order
3. Send to the statistics calculator with a group by on the newly created month year attribute
4. Use the cumulative output (be aware that there may be a workaround required to get round an issue with the statistics calculator)

https://knowledge.safe.com/questions/102959/incorrect-results-from-statisticscalculators-cumul.html

Thanks @ebygomm for the support but it is not adding the previous month . the output should be as rolling12month output

I misread, I think you don't need the group by at all in that case, and then just keep the output for the first of each month