import javafx.scene.control.DatePicker;
import java.time.Instant;
import java.time.LocalDate;
import java.time.ZoneId;
import java.util.Date;

@FXML
private DatePicker datePicker;

@FXML
private void handleButtonAction(ActionEvent event) {
    // your code here
    LocalDate localDate = datePicker.getValue();
    Instant instant = Instant.from(localDate.atStartOfDay(ZoneId.systemDefault()));
    Date date = Date.from(instant);
    // your code here
}