Converting dates in a feature collection to a previous date

Question

I have a featureCollection with the dates for 2021. I would like to do a date conversion for this featureCollection to get the same dates in 2017. I have written the function above but it returns the error. How can I solve this?

//Date conversion
print(swarms)

//Converting data parameter 

function epoch2date(feature){
  var epoch = feature.get('STARTDATE');
  var date = ee.Date(epoch).format('YYYY-MM-dd');
  return feature.set('date', date);
}

var swarmDates = swarms.map(epoch2date);
print(swarmDates)


//Compute dates in 2017
function dates(feature){
  var date = swarmDates.get('date');
  var newDate = ee.Date(date).advance(-4, 'year').format('YYYY-MM-dd');
  return feature.set('newDate', newDate)
}

var dates2017 = swarms.map(dates);

print(dates2017)

The link to the code to is;https://code.earthengine.google.com/098d3ae0185833f573d8f3e372fc5160

Answer 1

You have two typos in your code. In the function dates your are mistakenly calling swarmDates instead of feature. And for dates2017 you have to call map on swarmDates instead of swarms

//Compute dates in 2017
function dates(feature){
  var date = feature.get('date');
  var newDate = ee.Date(date).advance(-4, 'year').format('YYYY-MM-dd');
  return feature.set('newDate', newDate)
}

var dates2017 = swarmsDates.map(dates);

print(dates2017)