At the time you are saving the rest of the data, you can save the date in a separate column. If you are not sure how to get the current date with JavaScript, you can reference online resources on JavaScript. Here's one example: https://stackoverflow.com/questions/1531093/how-do-i-get-the-current-date-in-javascript?rq=1

If you use this approach, your trigger will look something like this:

var today = new Date();

var dd = String(today.getDate()).padStart(2, '0');

var mm = String(today.getMonth() + 1).padStart(2, '0'); //January is 0!

var yyyy = today.getFullYear();

today = mm + '/' + dd + '/' + yyyy;

SheetsSet('field',today);