Hi Romain,
so you would need to get the last day of the previous month.
Therefore first need the Time Generator (1 Row, use execution time)
Than use the Extract date field node to get number of days (today would be 12)
Multiply this number with -1 (Math node)
Use the date time shift node and substract the number of days
Use the Extract date field node again, and you are done
Let me know if this works.
Best, Iris