What is the impact of using #non thread-safe class? - thread-safety

Question

I have written the following code, as I was trying to understand the situation where I would use ThreadLocals:-
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
public class TestDF {
private DateFormat df = new SimpleDateFormat("MM/dd/yy");
public String formatCurrentDate() {
System.out.println(">>>>>>>"+Thread.currentThread().getName());
Date d = new Date();
return df.format(d);
}
public String formatFirstOfJanyary1970() {
System.out.println(">>>>>>>" + Thread.currentThread().getName());
Date d = new Date(0);
return df.format(d);
}
public static void main(String[] args) {
TestDF df = new TestDF();
Thread t1 = new Thread(new WorkerThread(df));
Thread t2 = new Thread(new WorkerThread1(df));
t1.start();
t2.start();
}
public static class WorkerThread implements Runnable {
TestDF df;
public WorkerThread(TestDF df) {
this.df = df;
}
#Override
public void run() {
Thread.currentThread().setName("Worker-Thread 1");
System.out.println("Inside Thread1*");
System.out.println("Thread 1 "+df.formatCurrentDate());
System.out.println("Inside Thread1**");
try {
Thread.sleep(5000l);
System.out.println("Inside Thread1***");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static class WorkerThread1 implements Runnable {
TestDF df;
public WorkerThread1(TestDF df) {
this.df = df;
}
#Override
public void run() {
Thread.currentThread().setName("Worker-Thread 2");
System.out.println("Inside Thread2*");
System.out.println("Thread 2 "+df.formatFirstOfJanyary1970());
System.out.println("Inside Thread2**");
try {
Thread.sleep(5000l);
System.out.println("Inside Thread2***");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
The out which I am receiving is as follows:-
Inside Thread1*
Inside Thread2*
>>>>>>>Worker-Thread 2
>>>>>>>Worker-Thread 1
Thread 1 06/09/15 <--
Thread 2 06/09/15 <--
Inside Thread1**
Inside Thread2**
Inside Thread1***
Inside Thread2***
I am aware that SimpleDateFormat is not thread-safe; but still couldn't make out how is happening.