I have a field that is a date. It’s written: month/day/year. I have redefined this field as _time.
So, I am able to count by month, however, when I want to see only the result for last month it didn’t work.
The time range that you applied on second query (using earliest) is only applicable to _time before your overwrite it with FirstDate field. It seems like the data from Active.csv in last 1 month, doesn't contains any dates for previous month. Give this a try