The invoices are dated the first day of the billing period (e.g. April 1st for the period of 4/1 through 4/30) when it is standard to date an invoice the last day of the billing period.
We annotate invoices based on the billing period. For example invoice issued on Jan 1st 2020 would state 'Final invoice for the December 2019 billing period' and the issue date for that invoice would state Jan 1st, 2020 (as that is when the invoice is generated and sent to users). We hope this clarifies any confusion with invoice dates.