gistfile1.txt

var
  minReturn = Infinity,
  minReturnStartYear,
  timeSpan = 32;
for (var startYear = 1871; startYear < 2014 - timeSpan; startYear ++) {
  document.getElementsByName('y1')[0].value = startYear;
  document.getElementsByName('y2')[0].value = startYear + timeSpan;
  doCalc();
  var currentReturn = +document.getElementsByName('fv')[0].value;
  if (currentReturn < minReturn) {
    minReturn = currentReturn;
    minReturnStartYear = startYear;
  }
}
console.log("The minimum return for $1 was $" + minReturn + " from " + minReturnStartYear + " to " + (minReturnStartYear + timeSpan));