A side conversation arising from this thread,
… reminded me that if stuck, think of the issue as a maths problem. So, rather than ‘last month’ think of ‘this month minus one month’. Doing so might make it easier to see which Date-type action code operators and keywords are pertinent to your task. Remember, the software doesn’t ‘understand’ human dates in the way we do, so the less we cause the code to guess our intent, the less scope for error.
And in the context of month, we may think of hem as January, Février, or März, etc. But in the Western calendar they are also months 1 through 12 regardless of their local name. Also makes the above maths easer to think through.