Meaning of "advance(-2, 'month')" in GEE

Question

I am learning GIS using Google Earth Engine.

When learning "Changing detection" I have this code:

// Define the area of interest
var geometry = ee.Geometry.Polygon([[
 [76.37639666685044, 11.766523239445169],
 [76.37639666685044, 11.519036946599561],
 [76.78426409849106, 11.519036946599561],
 [76.78426409849106, 11.766523239445169]
]]);
var fireStart = ee.Date('2019-02-20');
var fireEnd = ee.Date('2019-02-25');
Map.centerObject(geometry, 10)
var s2 = ee.ImageCollection("COPERNICUS/S2")
// Write a function for Cloud masking
function maskS2clouds(image) {
 var qa = image.select('QA60')
 var cloudBitMask = 1 << 10;
 var cirrusBitMask = 1 << 11;
 var mask = qa.bitwiseAnd(cloudBitMask).eq(0).and(
 qa.bitwiseAnd(cirrusBitMask).eq(0))
 return image.updateMask(mask)//.divide(10000)
 .select("B.*")
 .copyProperties(image, ["system:time_start"])
}
// Apply filters and cloud mask
var filtered = s2
 .filter(ee.Filter.bounds(geometry))
 .map(maskS2clouds)
 .select('B.*')
// Create Before and After composites
var before = filtered
 .filter(ee.Filter.date(
 fireStart.advance(-2, 'month'), fireStart))
 .median()
var after = filtered
 .filter(ee.Filter.date(
 fireEnd, fireEnd.advance(1, 'month')))
 .median()

I don't understand what is the mean of "fireEnd.advance(1, 'month')" and "fireStart.advance(-2, 'month') "line.

What does it mean by "-2" or "1"?

Answer 1

Its a time Shift!

In your Code:

fireStart.advance(-2, 'month')

fireStart is a Date. With advance you can shift the Date. The examples output ist the day just 2 Months before fireStart.

Just watch here for more Info.

For Future EarthEngine Questions you should ask Google or look directly into the API Reference. Just search for "EarthEngine" + Keyword (e.g. .advance)