It should say up to and including... on the left hand side.
It is current calendar month, usually to yesterday.
So if you check it on the 1st day of the month you'll get your usage for the previous month. Or check it on the last day of the month for close enough!
BT ADSL customer getting 1.9 Mbps on a new road / new build development
CAB not FTTC enabled, not part of the 66% commercial plan. Not rural - no BDUK funding
(Virgin Media nearby)