Converting from GMT to Local Timezone using two seperate fields (date & time).

[John Foley]

Hello everyone. I've been working on this for a while today and I've hit a wall as to what the issue is.

Basically, I need to convert to a local time from GMT time. I am using a Calendar object, (which will contain the time in GMT format that I need to convert to local time). I am using a date (String) formatted as yyyy-MM-dd that I need to use in the conversion process. I will also have the timezone I need to convert to (IE America/Chicago etc...). I've posted my code below but here is a summary of what I'm trying to do:

I need to convert the time in the Calendar Object to local time using the timezone I have and return a String with the time formated as HHmm. What I've been trying to do is to build a Date object by using SimpleDateFormat.parse() that will contain the date from the string, and the time from the calendar. I will then use SimpleDateFormat to format that date into the proper String with the correct converted local time. However, when I run this, it doesn't seem to convert my time. It looks like it's not using the Timezone I provide it, but keeps it at the default CENTRAL time (my local machine).

I pull the date String and build the SimpleDateFormat object as "yyyy-MM-ddTHHmm". After I build the SimpleDateFormat object, I set it's timezone to something like America/Los_Angeles and I'll put 0900 in my calendar and use 2010-03-05 for the date. I would exepect my 0900 to change to 0700 after my conversion, however it doesn't. It stays at 0900. I hope i've explained everything clearly enough, please let me know if I need to elaborate anymore.

    
private String[] processApptDateAndTime(Calendar value, String custTimeZone, String custDate) throws Exception
	{
		SimpleDateFormat sdfDateTime = new SimpleDateFormat("yyyy-MM-dd'T'HHmm");
		SimpleDateFormat sdfDate = new SimpleDateFormat("yyyy-MM-dd");
		SimpleDateFormat sdfTime = new SimpleDateFormat("HHmm");
		
		String[] dateTime = {FieldValueConstants.NULL_VALUE, FieldValueConstants.NULL_VALUE};
		System.out.println("Calendar to String Before: "+value.toString());
		TimeZone tz = value.getTimeZone();
		Date date = value.getTime();
		Date finalDate = date;
		 //  Pulling TimeZone from property file based on tag.
		TimeZone zone = FieldUtil.retrieveTimeZone(custTimeZone); <- Setting TimeZone here to America/Los_Angeles
								
		StringBuffer sb = new StringBuffer();
		Date apptDateTime;
						
		sb.append(custDate).append("T").append(sdfTime.format(date));
		try
		{
			sdfDateTime.setTimeZone(zone);
			apptDateTime = sdfDateTime.parse(sb.toString());
			finalDate = apptDateTime;  
			System.out.println("ApptDateTime: "+apptDateTime.toString());
		}
		catch (Exception e)
		{
			throw e;
		}
		
		//finalDate = dateTest;
		sdfDate.setTimeZone(zone);
		sdfTime.setTimeZone(zone);
		
		dateTime[0] = sdfDate.format(finalDate);
		dateTime[1] = sdfTime.format(finalDate);<- I would expect the Time to be converted correctly here
		
		return dateTime;
	}

Thank you.

Edited to remove un-necessary parts in code - can add them back if needed.

[John Foley]

I've been playing around with this some more and have more details. I can see where the problem is, but I don't know what is causing it.


Can anyone explain why when I parse the new Date Object below (located in 2nd code structure below on this line

apptDateTime = sdfDateTime.parse(sb.toString());

: , my time changes from 09:01 to 11:01? The conversion to Pacific time looks to be working correctly it's just converting it from 11:01 and not the 09:01.

Here is the code I'm using to build my test case (With notes):

    
public Calendar buildInputs()
{
		SimpleDateFormat dateTime = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm");
		SimpleDateFormat sdfTime = new SimpleDateFormat("HH:mm");
		Calendar cal = Calendar.getInstance();
		String apptStart = "09:01";
	
		try
		{
			Date start = sdfTime.parse(apptStart);
		
			TimeZone gmt = cal.getTimeZone();  //<-- At this point, the TimeZone is CST (my local) and the time is still 09:01
			
			cal.setTimeZone(gmt.getTimeZone("GMT")); //<-- I convert calendar to GMT
			cal.setTime(start);   //<-- and after setting the calendar with new time, it is now 03:01, which I'm expecting. 
		}
		catch(Exception e)
		{
			System.out.println(e.toString());
		}
		
				
		
		return cal;
	}

I then pass that calendar to my method which also takes in a date field to perform the logic to convert this to local time (in this example Pacific). Here is the code which does the conversion (with notes and findings):

   
private String[] processApptDateAndTime(Calendar csiCal, String custTimeZone, String custDate) throws Exception
	{
		SimpleDateFormat sdfDateTime = new SimpleDateFormat("yyyy-MM-dd'T'HHmm");
		SimpleDateFormat sdfDate = new SimpleDateFormat("yyyy-MM-dd");
		SimpleDateFormat sdfTime = new SimpleDateFormat("HHmm");
				
		String[] dateTime = {"NULL_VALUE", "NULL_VALUE"};
		System.out.println("Calendar to String Before: "+csiCal.toString());
		TimeZone tz = csiCal.getTimeZone();    //<-- At this point tz for calendar is GMT which is what I expect...
		Date date = csiCal.getTime();  //<-- after pulling date from calendar the time is back to 09:01 and the timeZone is CST.
		Date finalDate = date;

		if ("GMT".equalsIgnoreCase(tz.getID()))
		{
			if (isStringPopulated(custTimeZone))
			{
				
				TimeZone zone = retrieveTimeZone(custTimeZone);  //<--retrieveTimeZone returns timeZone object set to "America/Los_Angeles."  I'm trying to convert to this timeZone from GMT
								
				if (!"GMT".equalsIgnoreCase(zone.getID()))
				{
					
					if (isStringPopulated(custDate))
					{
						StringBuffer sb = new StringBuffer();
						Date apptDateTime;
						
						// Using the customer date and the time passed to us we built a dateTime format - yyyy-MM-dd'T'HHmm
						sb.append(custDate).append("T").append(sdfTime.format(date));
						try
						{
							sdfDateTime.setTimeZone(zone);
							apptDateTime = sdfDateTime.parse(sb.toString());  //<-- For some Reason at this point in time, when I parse the StringBuffer which contains 
//"2010-03-05T09:01, the new Date Object time changes to 11:01 CST.  I think this is where my problem is.
							
							finalDate = apptDateTime;
							System.out.println("ApptDateTime: "+apptDateTime.toString());
						}
						catch (Exception e)
						{
							throw e;
						}
					}
					
					
					sdfDate.setTimeZone(zone);
					sdfTime.setTimeZone(zone);  //<-- Setting the SimpleDateFormat timeZone to America\Los_Angeles.
							
					dateTime[0] = sdfDate.format(finalDate);
					dateTime[1] = sdfTime.format(finalDate);  //<-- When I format it to the string array, it comes back as 09:01.  
				}
			}
		}
		else
		{
			sdfDate.setTimeZone(csiCal.getTimeZone());
			sdfTime.setTimeZone(csiCal.getTimeZone());
			
			dateTime[0] = sdfDate.format(finalDate);
			dateTime[1] = sdfTime.format(finalDate);
		}
		
		return dateTime;
	}

[Paul Clapham]

That was very long and very wide so I didn't read all of it. But you seem to be under the impression that a Date has a time zone. It doesn't. Your comments suggest you are displaying a Date in some way and expecting that display to use some time zone other than your local time zone. It won't.

If you have a string which represents a timestamp in one time zone, and you want to change that to a string which represents a timestamp in another time zone, here's what you have to do:

Create a SimpleDateFormat and apply the first time zone to it

Use that to parse the String to a Date object

Create another SimpleDateFormat and apply the second time zone to it

Use that to format the Date object to a new String

Basically that's it. You don't need anything near as complicated as what you had there. In particular I don't think that Calendar objects are going to help.

[John Foley]

Thanks for the reply Paul, I'll shorten everything down to where the issue is. Unfortunately, I have to use the Calendar object because that's what is being given to me.

I need to get the time from the calendar and that will give me the Date Object.

I then use SimpleDateFormat on the new date object to get me the time as "HHmm". I create a String buffer to concatenate the date I'm given (String) along with the time(hours and minutes) I just pulled out (via SimpleDateFormat) to parse a new date object.

the string buffer will have this value: "2010-03-05T0901".

I will then use another SimpleDateFormat object setup as "yyyy-MM-dd'T'HHmm" to parse the StringBuffer I just created above into a new date object.

Now in debug mode I can see the StringBuffer is set to 2010-03-05T0901, however, after I parse that into another date object, Debug is showing the time for that object as 11:01, which doesn't look right. I'm not sure where the extra 2 hours are coming from. I would think that the time in the date object would stay at 0901. Am I misunderstanding something?

Thanks,

Date date = csiCal.getTime();

StringBuffer sb = new StringBuffer();
Date apptDateTime;
						
// Using the customer date and the time passed to us we built a dateTime format - yyyy-MM-dd'T'HHmm
sb.append(custDate).append("T").append(sdfTime.format(date));
try
{
	sdfDateTime.setTimeZone(zone);
	apptDateTime = sdfDateTime.parse(sb.toString());  //<-- For some Reason at this point in time, when I parse the StringBuffer which contains 
//"2010-03-05T09:01, the new Date Object time changes to 11:01 CST.  I think this is where my problem is.
							
	finalDate = apptDateTime;
	System.out.println("ApptDateTime: "+apptDateTime.toString());
}
catch (Exception e)
{
	throw e;
}

[John Foley]

So I guess the short question is, when I use SimpleDateFormat(yyyy-MM-dd'T'HHmm) to parse "2010-03-05T0901" the date I get back is "Fri Mar 05 11:01:00 CST 2010" and not "Fri Mar 05 09:01:00 CST 2010"?