Java timezone - strange behavior with IST?

3
votes

I have the below code: 

DateFormat df = new SimpleDateFormat("M/d/yy h:mm a z");
df.setLenient(false);
System.out.println(df.parse("6/29/2012 5:15 PM IST"));

Assuming I now set my PC's timezone to Pacific Time (UTC-7 for PDT), this prints

Fri Jun 29 08:15:00 PDT 2012

Isn't PDT 12.5 hours behind IST (Indian Standard Time)? This problem does not occur for any other timezone - I tried UTC, PKT, MMT etc instead of IST in the date string. Are there two ISTs in Java by any chance?

P.S: The date string in the actual code comes from an external source, so I cannot use GMT offset or any other timezone format.

4
javatimezone
There are several ISTs as far as I know. Judging by the time difference (9 hours), you've probably got Israel Standard Time. - biziclop
@biziclop I thought about this. But what veered me away from that was the below: System.out.println(TimeZone.getTimeZone("IST").getRawOffset()); This prints 19800000, or 5.5 hours, which suggests that it is indeed Indian Standard Time. Or is it picking up the first of many timezones with the same ID "IST"? If so, how can something be an "ID" if it is the same for many things? - Vasan
@esej It's more likely to be an ambiguity in the timezone abbreviations: en.wikipedia.org/wiki/List_of_time_zone_abbreviations - biziclop
Looking at the source code it seems that IST is India... - assylias
@Vasan it's not really an ID and SimpleDateFormat is a strange beast, it's very likely that it isn't using TimeZone.getTimeZone() at all. - biziclop

4 Answers

11
votes

The abberviated names of timezone are ambiguous and have been deprecated for Olson names for timezones. The following works consistently as there may be be differences in the way parse() and getTimezone() behaves. 

SimpleDateFormat sdf = new SimpleDateFormat("M/d/yy h:mm a Z");
TimeZone istTimeZone = TimeZone.getTimeZone("Asia/Kolkata");
Date d = new Date();
sdf.setTimeZone(istTimeZone);
String strtime = sdf.format(d);
6
votes

Sorry, I have to write an answer for this, but try this code:

public class Test {

    public static void main(String[] args) throws ParseException {
        DF df = new DF("M/d/yy h:mm a z");
        String [][] zs = df.getDateFormatSymbols().getZoneStrings();
        for( String [] z : zs ) {
            System.out.println( Arrays.toString( z ) );
        }
    }

    private static class DF extends SimpleDateFormat {
        @Override
        public DateFormatSymbols getDateFormatSymbols() {
            return super.getDateFormatSymbols();
        }

        public DF(String pattern) {
            super(pattern);
        }
    }

}

You'll find that IST appears several times in the list and the first one is indeed Israel Standard Time.

4
votes

Not an answer but see the output + code below - it does seem that parse treats IST differently from TimeZone.getTimeZone("IST")...

Fri Jun 29 16:15:00 BST 2012
Fri Jun 29 12:45:00 BST 2012
Fri Jun 29 12:45:00 BST 2012
*BST = London

public static void main(String[] args) throws InterruptedException, ParseException {
    DateFormat fmt1 = new SimpleDateFormat("M/d/yy h:mm a Z");
    Date date = fmt1.parse("6/29/2012 5:15 PM IST");
    System.out.println(date);

    DateFormat fmt2 = new SimpleDateFormat("M/d/yy h:mm a");
    fmt2.setTimeZone(TimeZone.getTimeZone("IST"));
    System.out.println(fmt2.parse("6/29/2012 5:15 PM"));

    DateFormat fmt3 = new SimpleDateFormat("M/d/yy h:mm a");
    fmt3.setTimeZone(TimeZone.getTimeZone("Asia/Kolkata"));
    System.out.println(fmt3.parse("6/29/2012 5:15 PM"));
}
0
votes

It is because of IST will have multiple meanings like Irish Standard Time, Isreal Standrad Time, Indian Standard Time.

Ref: https://www.timeanddate.com/time/zones/

Use setTimezone() method specifically to set the timezone.

Ex: parser.setTimeZone(TimeZone.getTimeZone("specify timezone in detail here"));