How to use the method calendar.getTimeInMillis() and setTimeInMillis(bb)

Question

I have met some problems in writing my programme,I want to know what the time is after every 20 minutes so have written some codes as below ,but the compilar warned me that there are some errors in them ,and I don't know how to use them correctly ,can you help me?
Thank you very much!
the code is:
begin=datime+" 00:00:00";
Calendar calendar=Calendar.getInstance();
DateFormat df = DateFormat.getDateInstance();
Date start=df.parse(begin);
calendar.setTime(start);
long aa=calendar.getTimeInMillis();
calendar.setTimeInMillis(aa);

Answer 1

Hopefully the following snippet of code will help you out (note that the Date class seems more appropriate for what you need to do):
import java.util.*;
import java.text.*;
      String begin="00:00:00";
      SimpleDateFormat df = new SimpleDateFormat("HH:mm:ss");
      df.setTimeZone(TimeZone.getTimeZone("GMT"));
      Date start=new Date();
      try {
         start=df.parse(begin);
      } catch (Throwable e) {
         System.out.println("Invalid time format");
      long aa=start.getTime();   // to get the time in milliseconds
      System.out.println(aa);    // just to see what the answer is (should be zero)
      start.setTime(aa);         // to set the time in milliseconds
...V.V.