Get data for previous month in postgresql

Subtract one month from the current month, then “truncate” that to the beginning of that date. As you don’t want to include rows from “this” month, you also need to add a condition for that

SELECT *
FROM Conference 
WHERE date_start >= date_trunc('month', current_date - interval '1' month)
  and date_start < date_trunc('month', current_date)

date_trunc('month', current_date - interval '1' month) will return the 1st day of the previous month and date_trunc('month', current_date) will return the first day of “this” month.