SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Date birth = sdf.parse("2000-01-01");
Date now = new Date(System.currentTimeMillis());
Calendar c = Calendar.getInstance();
c.setTimeInMillis(now.getTime() - birth.getTime());
int y = c.get(Calendar.YEAR)-1970;
int m = c.get(Calendar.MONTH);
int d = c.get(Calendar.DAY_OF_MONTH)-1;

SimpleDateFormat myFormat = new SimpleDateFormat("dd MM yyyy");
String classStartData = "31 01 2021";
String classEndData = "08 03 2021";
Date dateClassStart = myFormat.parse(classStartData);
Date dateClassEnd = myFormat.parse(classEndData);
long differenceWeek = dateClassEnd.getTime() - dateClassStart.getTime();
int programLength = (int)(TimeUnit.DAYS.convert(differenceWeek, TimeUnit.MILLISECONDS)/7);
System.out.println("Class length in weeks: " +programLength);