GEE copy system:time_start" to formulas

Question

How can I copy "system:time_start" to other formulas from collection?

Like EVI:

var evi2020 = med2020.expression( '2.5 * ((NIR - RED) / (NIR + 6 * RED - 7.5 * BLUE + 1))', { 'NIR': med2020.select('B8'), 'RED': med2020.select('B3'), 'BLUE': med2020.select('B2') }).rename('EVI2020');

or to other formula:

var formula = image.expression( '(ndvi2019 - ndvi2018) / mean', { 'ndvi2019': image.select('ndvi2019'), 'ndvi2018': image.select('ndvi2018'), 'mean': image.select('mean') }).rename('formula');

Answer 1

You can use .copyProperties(image, ['system:time_start']). An example below shows how to implement this command:

var addEVI = function(image) {
  return image.expression(
    '2.5 * ((NIR - RED) / (NIR + 6 * RED - 7.5 * BLUE + 1))', {
      'NIR': image.select('B5'),
      'RED': image.select('B4'),
      'BLUE': image.select('B2')})
      .rename('EVI')
      .float()
      .copyProperties(image, ['system:time_start']);
};

In this example you create a function to add EVI values to each image in an image collection.