How to make dimension show prev month dates

[#1]

Hi All,

how do i create a calculated dimension for dates to show for prev month if we on current  max month

each month it must roll back by a full prev month.

or in the Expression if possible also, if maybe we want to sum sales.

e.g

Dates
2015/09/30
2015/09/28
2015/08/12
2015/08/06
2015/08/04
2015/07/17

output should be:

Dates
2015/08/12
2015/08/06
2015/08/04

[Gysbert_Wassenaar]

If(Dates<MonthStart(today()) and Dates>=MonthStart(today(),-1),Dates)