Google Earth engine (GEE) - use sentinel-2 data acquisition to obtain the NDVI difference of one month ago (Guatemala as an example)

This experiment is mainly through sentinel2 Data monitoring and analysis before and after changes in a region . The results were analyzed by comparing the results before and after one month . Look for differences in one month

Code ：

// Study area vector , It is suggested that you change your research area 
var table = ee.FeatureCollection("projects/essential-rider-326809/assets/Guatemala");

// 1. Define areas of interest .
//  The map window at the bottom , Use the draw button on the left , Draw a polygon or rectangle around the area of interest .
//  This will create a file named  "geometry " The variable of , It will appear at the top of the code editor window  "Imports " Under a .

//  Define date range , To generate the greenest pixels before and after the event 、 Cloudless composite image .
//  If the event happened in summer , This can be done within one month before and after the landslide event .
//  perhaps , If the event happens in other seasons , Green vegetation peak is used （ And minimum shadow ） A few months of images .
//  for example ,PRE = 2019 year 6 month ,POST = 2020 year 6 month 

        //  A month ago 
        //  Defining time 
        var pre_start = ee.Date('2019-07-01');
        var pre_end = ee.Date('2019-07-31');
        
        // POST-EVENT IMAGE
        //  After a month 
        var post_start = ee.Date('2019-08-01');
        var post_end = ee.Date('2019-09-01');
        
// 3.  Set the name of the Google hard drive folder where you want to export pictures . Or some other name