SteamGuardAccount.cs

public string GenerateSteamGuardCodeForTime(long time) {
  if(this.SharedSecret == null || this.SharedSecret.Length == 0) {
    return "";
  }

  Console.WriteLine(time);

  byte[] sharedSecretArray = Convert.FromBase64String(this.SharedSecret);
  byte[] timeArray = new byte[8];

  time /= 30L;

  for(int i = 8; i > 0; i--) {
    timeArray[i - 1] = (byte) time;
    time >>= 8;
  }

  HMACSHA1 hmacGenerator = new HMACSHA1();
  hmacGenerator.Key = sharedSecretArray;
  byte[] hashedData = hmacGenerator.ComputeHash(timeArray);
	
  byte[] codeArray = new byte[5];
  try {
    byte b = (byte) (hashedData[19] & 0xF);
    int codePoint = (hashedData[b] & 0x7F) << 24 | (hashedData[b + 1] & 0xFF) << 16 | (hashedData[b + 2] & 0xFF) << 8 | (hashedData[b + 3] & 0xFF);

    int k = 0;

    for(int i = 0; i < 5; ++i) {
      codeArray[i] = steamGuardCodeTranslations[codePoint % steamGuardCodeTranslations.Length];
      codePoint /= steamGuardCodeTranslations.Length;
    }
  } catch(Exception) {
    return null; //Change later, catch-alls are bad!
  }
  return Encoding.UTF8.GetString(codeArray);
}

SteamGuardAccount.js

  generateSteamGuardCodeForTime(time) {
    if (!this.sharedSecret || this.sharedSecret.length === 0) return "";

    time /= 30;

    let timeArray = new Uint8Array(8);

    for (let i = 8; i > 0; i--) {
      timeArray[i - 1] = time;
      time >>= 8;
    }
    timeArray = timeArray.buffer;
    timeArray.length = timeArray.byteLength;

    let key = CryptoJS.enc.Base64.parse(this.sharedSecret);

    let wordArray = CryptoJS.enc.u8array.parse(timeArray);

    let hexHashedData = CryptoJS.HmacSHA1(wordArray, key).toString();
    let codeArray = new Int8Array(5);

    let hashedData = [];
    for (let i = 0; i < hexHashedData.length / 2; i++) {
      hashedData[i] = parseInt(hexHashedData.substr(i * 2, 2), 16);
    }

    let b = Byte.cast(hashedData[19] & 0xF);
    let codePoint = (hashedData[b] & 0x7F) << 24 | (hashedData[b + 1] & 0xFF) << 16 | (hashedData[b + 2] & 0xFF) << 8 | (hashedData[b + 3] & 0xFF);

    for (let i = 0; i < 5; ++i) {
      codeArray[i] = this.steamGuardCodeTranslations[codePoint % this.steamGuardCodeTranslations.length];
      codePoint = Math.floor(codePoint / this.steamGuardCodeTranslations.length);
    }
    let encodedString = String.fromCharCode.apply(void 0, codeArray);
    return decodeURIComponent(escape(encodedString));
  }