Yes, I was just following your explicit requirement.
So have you found that NR can do that kind of thing? Let you pick the month?
You can make a dashboard and the viewer can change the time range themselves. So you can create a dashboard and let someone configure their view for the month they want assuming the month is still retained.
You do not need to know the last day of the month (but how hard is that?) You can do since the 1st until the 1st using the dates like you have above.
SELECT count(*) from Transaction Since '2021-06-01 00:00:00' until '2021-07-01 00:00:00' WITH TIMEZONE 'America/New_York'