Set first and last day of month in DatePicker

I'm having difficulty setting a start and end date for the current month in the datepicker on a custom page, I'd appreciate any help if possible.

Solution

$this->form->fill([
  'dataInicio' => ...,
  'dataFim' => ...
]);

$this->form->fill([
  'dataInicio' => ...,
  'dataFim' => ...
]);

$this->form->fill([
  'dataInicio' => ...,
  'dataFim' => ...
]);

$this->form->fill([
  'dataInicio' => ...,
  'dataFim' => ...
]);