Error on GEE: No features contain non-null values of "system:time_start"

Question

I'm calculating DVI over a region using MODIS. I am able to get the printout of the map properly but not the chart to retrieve the data. To calculate DVI, I did:

var dvi = img.expression(
    '(nir - red)',
    {
        red: img.select('sur_refl_b01'),    // 620-670nm, RED
        nir: img.select('sur_refl_b02'),    // 841-876nm, NIR
        blue: img.select('sur_refl_b03')    // 459-479nm, BLUE
      .rename('DVI')
    });

When I look at the inspector, I see the value for the DVI band, but the code for charting seems not to work. It says there's no system time.

var bandChart = ui.Chart.image.series(dvi, roi)

I also tried to add .filterDate ('2016-05-01', '2016-10-31') but it says the parameter is not a function. I.e.

var dvi = img.expression(
    '(nir - red)',
    {
        red: img.select('sur_refl_b01'),    // 620-670nm, RED
        nir: img.select('sur_refl_b02'),    // 841-876nm, NIR
        blue: img.select('sur_refl_b03')    // 459-479nm, BLUE
      .rename('DVI')
    });

I also tried to create a function (so as to specify timeframe) but that did not work either. How can I proceeed to get the chart?

var addDVI = function(image) {
  var dvi = img.expression(
    '(nir - red)',
    {
        red: img.select('sur_refl_b01'),    // 620-670nm, RED
        nir: img.select('sur_refl_b02'),    // 841-876nm, NIR
        blue: img.select('sur_refl_b03')    // 459-479nm, BLUE
      .rename('DVI')
      .filterDate('2016-05-01', '2016-10-31')

    });
    return image.addBands (dvi);
}; 

Answer 1

You need something like img.expression(...).copyProperties(img, 'system:time_start') (or use image if it's inside that function). When you do a computation on an image, the derived image does not retain the properties of the original image.