What you can do is, you can always keep the Relative Filter to Last 7 Months and add it to CONTEXT. So you are getting & MONTHS data. NOW TO Restrict the data to 10/31/2017 if you get it on 11/8/2017. Yoou add this calculation to Filter, your data would be Restricted to Last Month Always and it would be Dynamic. You need not to do any thing ever.
See Attached with Superstore.
Relative Date.twbx 1.2 MB
There are tow ways to anchor the date.
1. Using today()
2. Using the range of data set itself.
According to your statement, you can use option to determine the date range, because it seems like you get the data for October in several days after October ends and the Range seems only up to October.
Calculate the latest date of Data.
Then set filter based on the max date.
Hope this works.
Superstoredatefilter_SM_10.0.twbx 351.5 KB
Thanks for your help, it worked for me