Assume you have dates in columns C (index 3 in a 0-index 2d array) of a [[Google Sheets|Google Sheet]].
```javascript
// format dates to pass to client side
data = data.map(row => {
return row.slice(0,2).concat(
[
row[3].toLocaleString("en-us"),)
]).concat(row.slice(4))
});
```