I have written a basic code that can sum GDD for a specific date range for a single year as seen below:

var dataset = ee.ImageCollection('MODIS/006/MYD11A1')
                  .filter(ee.Filter.date('2017-04-15', '2017-05-20'));
var landSurfaceTemperature = dataset.select(['LST_Day_1km', 'LST_Night_1km']);

var tempToFahr = landSurfaceTemperature.map(function(image){
  var props = image.toDictionary(image.propertyNames());
  var Fahr = (image.multiply(0.02).subtract(273.15)).multiply(1.8).add(32);
  // Mask where one of Night or day temperature has no data
  var FahrMasks = Fahr.updateMask(Fahr.select('LST_Day_1km')).updateMask(Fahr.select('LST_Night_1km'));
  // we assume that the night temperature is the min temp, and the day temperature is the max temperature
  var meanFahr = FahrMasks.reduce('mean').rename('meanTemp');
  // Calculate the GGD and make all the negative values 0 (see: https://en.wikipedia.org/wiki/Growing_degree-day)
  var GDD = meanFahr.subtract(52.5).rename('GDD');
  var GDDnonNeg = GDD.where(GDD.lt(0), 0).rename('GDDnonNeg');
  return ee.Image(Fahr.addBands([meanFahr, GDD, GDDnonNeg]).setMulti(props));

// calculate the sum of GGD values
var summed = tempToFahr.select('GDDnonNeg').sum().rename('summedGDD');

var landSurfaceTemperatureVis = {
  min: 0,
  max: 15,
  bands: ['LST_Day_1km'],
  opacity: .6,
  palette: [
    '040274', '040281', '0502a3', '0502b8', '0502ce', '0502e6',
    '0602ff', '235cb1', '307ef3', '269db1', '30c8e2', '32d3ef',
    '3be285', '3ff38f', '86e26f', '3ae237', 'b5e22e', 'd6e21f',
    'fff705', 'ffd611', 'ffb613', 'ff8b13', 'ff6e08', 'ff500d',
    'ff0000', 'de0101', 'c21301', 'a71001', '911003'
Map.setCenter(-85.60371794450282,44.73590436363271, 10);
Map.addLayer(tempToFahr, landSurfaceTemperatureVis,'Land Surface Temperature', 1, 0.46);
Map.addLayer(summed.clip(features), {min: 0, max: 15, opacity: .6, palette: landSurfaceTemperatureVis.palette},'summed growing degree days', 1, 1);

// Export a cloud-optimized GeoTIFF.
  image: summed,
  description: 'imageToCOGeoTiffExample',
  scale: 1000,
  region: features,
  fileFormat: 'GeoTIFF',
  formatOptions: {
    cloudOptimized: true

This works great for a specific date range in a single year, however I cannot for the life of me figure out a a way to take multiple date ranges over multiple years and then average them onto a single layer.

One suggestion I had seen earlier proposed this code:

// 1. add years in each image
var tempToFahr_Y  = tempToFahr.map(function(f) {
  var date = ee.Date(f.get("system:time_start"))
  var year=  date.format("Y")
  return f.set({date: date, year: year})

// 2. get years in data
var years_list = tempToFahr_Y.reduceColumns(ee.Reducer.frequencyHistogram(), ["year"]).get("histogram")
var years_dic = ee.Dictionary(years_list).keys()

// 3. now map over the years dic
var tempToFahr_Y_sum = years_dic.map(function(x) { 
  var res= tempToFahr_Y.filter(ee.Filter.eq("year", x))
  var res_name = ee.Image(res).rename(ee.String("summedGDD_").cat(ee.String(x)))
  return res_name

// 4. into one imageCollection
var tempToFahr_Y_sum1 = ee.ImageCollection.fromImages(tempToFahr_Y_sum)

However, I cannot figure out how to enter multiple years and more importantly a specific date range within those years.

Any ideas?

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