DateDifferenceExample.pde

// https://stackoverflow.com/questions/20165564/calculating-days-between-two-dates-with-java/29812532

import java.util.Calendar;
import java.util.Date;
import java.util.Scanner;

void setup() {
  println(getDateDifference("04 03 2009", "12 06 2021", " "));
  println(getDateDifference("04 11 1999", "05 11 1999", " "));
}

void draw() {
  //
}

int getDateDifference(String date1, String date2, String separator) {
  String[] insert1 = date1.split(separator);
  String[] insert2 = date2.split(separator);

  Calendar cal = Calendar.getInstance();

  cal.set(Calendar.DAY_OF_MONTH, Integer.parseInt(insert1[0]));
  cal.set(Calendar.MONTH, Integer.parseInt(insert1[1]));
  cal.set(Calendar.YEAR, Integer.parseInt(insert1[2]));
  Date firstDate = cal.getTime();

  cal.set(Calendar.DAY_OF_MONTH, Integer.parseInt(insert2[0]));
  cal.set(Calendar.MONTH, Integer.parseInt(insert2[1]));
  cal.set(Calendar.YEAR, Integer.parseInt(insert2[2]));
  Date secondDate = cal.getTime();


  long diff = secondDate.getTime() - firstDate.getTime();

  return int(diff / 1000 / 60 / 60 / 24);
}