Copy system:time_start property in Google Earth Engine

Question

I'd like to copy the time:start property from dataset2021 collection to ndvi2021 and further to difference which is 2021-2020. How can I do it?

//point

var point = ee.Geometry.Point(-122.082, 37.42).buffer(50);

//Dataset
var dataset2021 = ee.ImageCollection('COPERNICUS/S2_SR')
                  .filterDate('2021-04-15', '2021-09-30')
                  .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 5))
                  .filterBounds(point) 
                  .select(['B4', 'B8']);
                  
var dataset2020 = ee.ImageCollection('COPERNICUS/S2_SR')
                  .filterDate('2020-04-15', '2020-09-30')
                  .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 5))
                  .filterBounds(point)
                  .select(['B4, 'B8']);

Map.centerObject(point, 14);

 // NDVI 2021
var med2021 = dataset2021.median();
var result2021 = med2021.select('B4', 'B8');
var ndvi2021 = result2021.normalizedDifference(['B8', 'B4']).rename('ndvi');

 // NDVI 2020
var med2020 = dataset2020.median();
var result2020 = med2020.select('B4', 'B8');
var ndvi2020 = result2020.normalizedDifference(['B8', 'B4']).rename('ndvi');

// difference
var diff2019 = ndvi2021.subtract(ndvi2020);

// CHART with properties

var chart = ui.Chart.image.series({
  imageCollection: ndvi2021.select('ndvi'),
  region: point,
  reducer: ee.Reducer.median(),
  scale: 10
}).setOptions({
  title: 'NDVI over time',
  hAxis: {title: 'Date'},
  vAxis: {title: 'NDVI'} 
  });

print(chart);


var chart = ui.Chart.image.series({
  imageCollection: diff2019.select('ndvi'),
  region: point,
  reducer: ee.Reducer.median(),
  scale: 10
}).setOptions({
  title: 'ZNDVI over time',
  hAxis: {title: 'Date'},
  vAxis: {title: 'difference'} 
  });

print(chart);

Answer 1

The value passed for parameter imageCollection is an image (ndvi2021 & diff2019) for the method ui.Chart.image.series(). Your imageCollection is reduced to a single image after applying median() function. It is not possible to show 'NDVI over time' for a single image.