You would have to update the Billing Overview plugin source code in order to set those dates. If you change them, they will be overridden when you upgrade Blesta so you will need to remember to re-add your changes back in afterward.
Open /plugins/billing_overview/controllers/admin_main.php
Look for the following toward the top of the file:
$dates = [
'today_start' => $this->Date->cast($datetime, 'Y-m-d 00:00:00'),
'today_end' => $this->Date->cast($datetime, 'Y-m-d 23:59:59'),
'month_start' => $this->Date->cast($datetime, 'Y-m-01 00:00:00'),
'month_end' => $this->Date->cast($datetime, 'Y-m-t 23:59:59'),
'year_start' => $this->Date->cast($datetime, 'Y-01-01 00:00:00'),
'year_end' => $this->Date->cast($datetime, 'Y-12-31 23:59:59')
];
Replace that with:
$month = $this->Date->format('n');
$new_fiscal_year = ($month >= 4);
$datetime_last_year = $this->Date->format('c', date('c', strtotime('now -1 year')));
$datetime_next_year = $this->Date->format('c', date('c', strtotime('now +1 year')));
$dates = [
'today_start' => $this->Date->cast($datetime, 'Y-m-d 00:00:00'),
'today_end' => $this->Date->cast($datetime, 'Y-m-d 23:59:59'),
'month_start' => $this->Date->cast($datetime, 'Y-m-01 00:00:00'),
'month_end' => $this->Date->cast($datetime, 'Y-m-t 23:59:59'),
'year_start' => $this->Date->cast(($new_fiscal_year ? $datetime : $datetime_last_year), 'Y-04-01 00:00:00'),
'year_end' => $this->Date->cast(($new_fiscal_year ? $datetime_next_year : $datetime), 'Y-03-31 23:59:59')
];
Save