As always, Maxim 1: Read the Documentation!
INTNX Function
If no fourth argument is specified, the aligment defaults to "BEGINNING", so with a month interval you always get the first of the month. Adding the DAY value and subtracting 1 will result in the same day one month back, with the peculiar twist of what happens when you are at the end of a month with more days than the previous month. If you apply this on July 31, you get July 1 instead of June 30 (which would be the result of INTNX with a SAME alignment) .
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