I'd hope that it's based on calendar months. Wouldn't invoices that span multiple years make things a lot more complicated in terms of accounting in general?
As far as UI/UX is concerned their tvOS app is in a really rough shape. The web UI has a fairly reasonable program guide (edit: for Live TV), while the app doesn't. Also navigation in the app doesn't feel polished at all unfortunately.
Didder is fantastic. I'm using it to convert images to grayscale for use with Waveshare e-ink displays. I've found no other tool that both lets you specify a custom palette (without which the resulting image looks pretty bad) and is reasonably quick.
For future reference, this works beautifully with their 9.7" display: didder --palette '000000 111111 222222 333333 444444 555555 666666 777777 888888 999999 aaaaaa bbbbbb cccccc dddddd eeeeee ffffff' -g -i - -o - edm --serpentine FloydSteinberg