check-in-label.php

function my_bfwc_filter_check_in_placeholder( $label ) {
  $label = 'Start rental';
  return $label;
}
add_filter( 'bfwc_filter_check_in_placeholder', 'my_bfwc_filter_check_in_placeholder' );

function my_bfwc_filter_check_out_placeholder( $label ) {
  $label = 'End rental';
  return $label;
}
add_filter( 'bfwc_filter_check_out_placeholder', 'my_bfwc_filter_check_out_placeholder' );