Convert a string to GregorianCalendar.

Question

I have a string from an email header, like Date: Mon, 27 Oct 2008 08:33:29 -0700. What I need is an instance of GregorianCalendar, that will represent the same moment. As easy as that -- how do I do it?

And for the fastest ones -- this is not going to work properly:

SimpleDateFormat format = ... // whatever you want
Date date = format.parse(myString)
GregorianCalendar calendar = new GregorianCalendar();
calendar.setTime(date)

because it will normalize the timezone to UTC (or your local machine time, depending on Java version). What I need is calendar.getTimeZone().getRawOffset() to return -7 * milisInAnHour.

Answer 1

I'd recommend looking into the Joda Time library, if that's an option. I'm normally against using a third-party library when the core platform provides similar functionality, but I made this an exception because the author of Joda Time is also behind JSR310, and Joda Time is basically going to be rolled into Java 7 eventually.

http://joda-time.sourceforge.net/

So anyway, if Joda Time is an option, something like this should work:

DateTimeFormatter formatter =
    DateTimeFormat.forPattern("your pattern").withOffsetParsed();
DateTime dateTime = formatter.parseDateTime("your input");
GregorianCalendar cal = dateTime.toGregorianCalendar();

I hope this helps.

Answer 2

And for the fastest ones -- this is not going to work properly ... because it will normalize the timezone to UTC (or your local machine time, depending on Java version). What I need is calendar.getTimeZone().getRawOffset() to return -7 * milisInAnHour.

Well technically this does work, because while it will return an object with TimeZone equal to the current system TimeZone, the time will be modified to account for the offset.

This code:

String dateString = "Mon, 27 Oct 2008 08:33:29 -0700";
DateFormat df = new SimpleDateFormat("E, dd MMM yyyy hh:mm:ss Z");
Date parsed = df.parse(dateString);
System.out.println("parsed date: " + parsed);

Calendar newCalendar = Calendar.getInstance();
newCalendar.setTime(parsed);

outputs:

parsed date: Mon Oct 27 11:33:29 EDT 2008

which technically is correct, since my system timezone is EDT / UTC minus four hours (which is three hours ahead of yours). If you express time as the number of milliseconds since January 1, 1970, 00:00:00 GMT (which is how the Date object stores it's date/time), then these date/times are equal, it's just the TimeZone that is different.

Your issue is really How do I convert a Date/Calendar into my timezone? For that, take a look at my response to the previous question How to handle calendar TimeZones using Java?