Each month beginning to end (1st to the 30th or 31st (Feb 28th or 29th))
I never noticed anything about the 20th of any month on mine. If it has a submit button, click it but only if you know you have reached the tier. There are times at the beginning of the month that show up wonky and add your entire year xx to the month.
edit, if you forgot to claim last months bucks you can still claim them. You can not claim forgotten months from year 1-12 as there is a deadline. The years start and end with March 1st.