title: No overload matches this call - moment js
body: <p>I have the following code:</p>
  <pre><code>const calibrationSpan = parent.CalibrationCycleID;
  const lastCalibrationDate = parent.LastCalibrationDate;
  
  const amountSpan: number = CycleCodes[calibrationSpan];
  
  const calibrationDueDate = moment(lastCalibrationDate).add(amountSpan, calibrationSpan[0]).toDate();
  
  console.log('span', calibrationSpan) 
  console.log('dues', calibrationDueDate)
  
  return calibrationDueDate;
  </code></pre>
  <p>It tells me No overload matches this call on the momentjs add function. If I log calibration span, it's a string from which I just grab the first char (d, m, y) .
  However if I have it like this</p>
  <pre><code>const calibrationDueDate = moment(lastCalibrationDate).add(amountSpan, &quot;d&quot;);
  </code></pre>
  <p>it runs as it should. any suggestion?</p>