Monthly Water Area Monitoring in Google Earth Engine?

Question

I've computed water area for each 8 days for about 20 years. I don't know how can I compute monthly area for these years? or How can I change 8 days product to the monthly in this code?

code link: https://code.earthengine.google.com/590d4cf09f90e591d69ed3a03039d4f7

Map.centerObject(geometry)

var modis = ee.ImageCollection('MODIS/006/MOD09A1')
.filterBounds(geometry)
.filterDate('2000-01-01','2020-01-01')
.select('sur_refl_b03','sur_refl_b02')
.map(function(img){

  var id = img.id();

  var scale = img.multiply(0.0001);
  var clip = scale.clip(geometry);
  var index = clip.normalizedDifference(['sur_refl_b03','sur_refl_b02']);

  var thr = index.gte(0.1);

  var mask = index.updateMask(thr);

 var area = mask.multiply(ee.Image.pixelArea().divide(1000000.0));

  return area
  .copyProperties(img,['system:time_start','system:time_end'])
});


print(modis);

var chart = ui.Chart.image.series(modis, geometry, 
ee.Reducer.sum(), 1000, 'system:time_start');

print(chart);