Google script to get currency rates from NBRB API.

NBRBRates.gs

function NBRBRates(currency, date) {
  var formattedDate = Utilities.formatDate(date, Session.getScriptTimeZone(), "yyyy-M-d");
  var url = Utilities.formatString("http://www.nbrb.by/API/ExRates/Rates/%s?onDate=%s&ParamMode=2", currency, formattedDate);
  var res = UrlFetchApp.fetch(url);
  res = JSON.parse(res);
  return res.Cur_OfficialRate;
}

// how to use
// =NBRBRates("EUR";TODAY())
// =NBRBRates("EUR";A1)