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Weirdly I didn't get an error when I ran your workflow (but I expected one).
In my original version I had used DateTimeParse, to convert your date into a proper date..but with your dates as they are this is no longer needed. I have updated the formula to reflect this.
In answer to your other question, I union'd the R & J outputs of the join because I wanted to make sure that I had an entry for every month regardless of whether there was an activity value for that month.