code: public static long getLastModifiedRecursive(File file) {
  		long time=file.lastModified();
  		if(file.isDirectory()) {
  			String[] list=file.list();
  			if(list!=null) {
  				int len=list.length;
  				for(int c=0; c<len; c++) {
  					long time2=getLastModifiedRecursive(new File(file, list[c]));
  					if (time2 > time) time=time2;
  				}
  			}
  		}
  		return time;
  	}
docs: Recursively gets the most recent modification time of a file or directory.