/// Labels for market-history session-half slot instants (UTC wire times).
String formatGuessSlotInstant(DateTime slotStart) {
  final DateTime utc = slotStart.toUtc();
  final String month = utc.month.toString().padLeft(2, '0');
  final String day = utc.day.toString().padLeft(2, '0');
  final String hour = utc.hour.toString().padLeft(2, '0');
  final String minute = utc.minute.toString().padLeft(2, '0');
  return '$month/$day $hour:$minute';
}

/// Active guess pair: older session half → next session half.
String formatGuessSlotRange({
  required DateTime slotStart,
  DateTime? newerSlotStart,
}) {
  if (newerSlotStart == null) {
    return '${formatGuessSlotInstant(slotStart)} UTC';
  }
  return '${formatGuessSlotInstant(slotStart)} – '
      '${formatGuessSlotInstant(newerSlotStart)} UTC';
}