Wgrywasz raporty od brokera.Dostajesz gotowy PIT-38.

// art. 11a: the rate from the last business
// day BEFORE the transaction closed.
async function rateForClose(currency, closedAt) {
  let day = previousDay(closedAt);

  for (let back = 0; back < 7; back++) {
    const rate = await nbpTableA(currency, day);
    if (rate) return rate;   // business day
    day = previousDay(day);  // weekend, holiday
  }
  return null;               // surfaced as a warning
}

// Every leg converted on its own rate,
// rounded to grosze before it is summed.
const profitPln = round2(trade.profit * rate.mid);
const swapPln = round2(trade.swap * rate.mid);
const feePln = round2(trade.fee * rate.mid);

revenue += Math.max(profitPln, 0);
costs += Math.abs(Math.min(profitPln, 0))
       + Math.abs(swapPln) + Math.abs(feePln);

// Two reports covering overlapping periods
// contain the same position twice.
const seen = new Set();

for (const trade of allFiles.flat()) {
  const key = fingerprint(trade); // ticket,
  // symbol, open + close time, volume
  if (seen.has(key)) { skipped++; continue; }
  seen.add(key);
  accepted.push(trade);
}

// art. 30b ust. 1, art. 30b ust. 1a and
// art. 30a are three separate buckets.
const ret = {
  cfd: settleCfd(trades),          // C / D
  crypto: settleCrypto(disposals), // E / F
  dividends: settleDividends(rows) // G
};

// No cross-netting: a CFD loss never reduces
// a gain on virtual currencies.