This does not work because your input (and your Formatter) do not have time zone information. A simple way is to parse your date as a LocalDate
first (without time or time zone information) then create a ZonedDateTime
:
public static ZonedDateTime convertirAFecha(String fecha) {
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd/MM/yyyy");
LocalDate date = LocalDate.parse(fecha, formatter);
ZonedDateTime resultado = date.atStartOfDay(ZoneId.systemDefault());
return resultado;
}