Woa! Why does Date/Calendar do this?

[Darrin Smith]

I have two Date objects. One holds just the date, and one holds just the time. You would THINK that adding the getTime of both of them would give you the date and time but it doesn't give you what you think!

Try this sample program:

  
 private void test1()
   {
      Date date = new Date();
      
      //I KNOW...DEPRECATED, but for this test go with it 
      date.setHours(0);
      date.setMinutes(0);
      date.setSeconds(0);
      
      String timeStr = "1630";
      
      Date time = getTime(timeStr);
      
      System.out.println("Date:" + date);
      System.out.println("Time: " + time);
 
      Date newDate = new Date(date.getTime() + time.getTime());
      
      System.out.println("NewDate:" + newDate);
 
      GregorianCalendar cal = new GregorianCalendar();
      cal.setTimeInMillis(date.getTime() + time.getTime());
 
      System.out.println("Cal:" + cal.getTime());
     
      
   }
 
private Date getTime(String timeStr) 
{
   Date date = new Date();
   
   //In 24 hour format
   try
   {
      date = FORMAT_24_HOUR.parse(timeStr);
   }
   catch (java.text.ParseException e)
   {
      System.out.println("PARSE PROBLEM");
   }
 
   return date;
}

You would think that you would get today's date with 1630 (4:30 pm) as the time...but you get 22:30. I think this has something to do with GMT, but given that none of the Date or Calendar objects show that (when you print them out they show you CST or CDT where I am).

Why does it add four hours like that?

Also, what is the most efficient way to get it corrected (to show 1630 instead of 2230)? Would it be just to add what I get from GregorianCalendar.get(Calendar.ZONE_OFFSET) to the date?

For example:

GregorianCalendar cal2 = new GregorianCalendar();
int offset = cal.get(Calendar.ZONE_OFFSET);
cal2.setTimeInMillis(date.getTime() + time.getTime() + offset);

Thanks!
[ October 05, 2006: Message edited by: Darrin Smith ]

[Tim LeMaster]

getTime
public long getTime()Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT represented by this Date object.

Returns:
the number of milliseconds since January 1, 1970, 00:00:00 GMT represented by this date.

using get time returns the number of milliseconds since the epoch in GMT.
So your code returns 21:30 in my EST timezone.

How bout something like this?

    
private void test2()
{
Calendar cal = Calendar.getInstance();
cal.set(Calendar.HOUR_OF_DAY, 0);
cal.set(Calendar.MINUTE, 0);
cal.set(Calendar.SECOND, 0);

DateFormat format = DateFormat.getDateTimeInstance();
System.out.println("Cal: " + format.format(cal.getTime()));

String timeStr = "1630";
String hour = timeStr.substring(0,2);
String minute = timeStr.substring(2,4);

cal.add(Calendar.HOUR_OF_DAY, Integer.parseInt(hour));
cal.add(Calendar.MINUTE, Integer.parseInt(minute));

System.out.println("Cal: " + format.format(cal.getTime()));
}

[Darrin Smith]

That works too as does using the Calendar.get(Calendar.ZONE_OFFSET. I think the Calendar.get might be the more efficient of the two so I'll probably stick to that.

Anyway, thanks for the reply. It may come in handy!
[ October 05, 2006: Message edited by: Darrin Smith ]

[Stan James]

Look into the TimeAndMoney and Joda Time packages. Java dates are pretty painful some times.