Incorrect time with FullName Timezone

Question

I am trying to convert time from timezone to another using fullname timezone. Below is the java api i have used.

    TimeZone.setDefault(TimeZone.getTimeZone("Africa/Algiers"));
    SimpleDateFormat formatter = new SimpleDateFormat("HH:mm:ss");
    String dateInString = "09:44:00";
    TimeZone tzGMT01 = TimeZone.getTimeZone("Africa/Algiers");
    formatter.setTimeZone(tzGMT01);

    Date dateInAmerica = formatter.parse(dateInString);
    String fromInput = formatter.format(dateInAmerica);
    TimeZone tzGMT03 = TimeZone.getTimeZone("Asia/Bahrain");
    formatter.setTimeZone(tzGMT03); 

    String toInput = formatter.format(dateInAmerica);
    Time ts = java.sql.Time.valueOf(toInput);

The expected output for time value of 09:44:00 converted to "Asia/Bahrain" time is 11:44:00. However the output is 13:44:00.

Can someone please guide what is wrong in the above statements.

Answer 1

Your problem is that a Java Date is a full date value, not just a time value. When you don't give year/month/day values, they default to 1970/01/01.

The world was different back in 1970. Algiers was using UTC+0 timezone, and Bahrain was using UTC+4, which is unlike today, where Algiers is using UTC+1 timezone, and Bahrain is using UTC+3.

Ignoring date will also cause issues for timezones that has Daylight Savings Time.

As you can see, date matters.

Here is code to show it:

TimeZone tzAlgiers = TimeZone.getTimeZone("Africa/Algiers");
TimeZone tzBahrain = TimeZone.getTimeZone("Asia/Bahrain");

SimpleDateFormat fmtTimeAlgiers = new SimpleDateFormat("HH:mm:ss");
fmtTimeAlgiers.setTimeZone(tzAlgiers);

SimpleDateFormat fmtDatetimeAlgiers = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
fmtDatetimeAlgiers.setTimeZone(tzAlgiers);

SimpleDateFormat fmtFullAlgiers = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss z (Z)");
fmtFullAlgiers.setTimeZone(tzAlgiers);

SimpleDateFormat fmtFullBahrain = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss z (Z)");
fmtFullBahrain.setTimeZone(tzBahrain);

Date date1970 = fmtTimeAlgiers.parse("09:44:00");
System.out.println(fmtTimeAlgiers.format(date1970) + "                          " + tzAlgiers.getID());
System.out.println(fmtFullAlgiers.format(date1970) + "   " + tzAlgiers.getID());
System.out.println(fmtFullBahrain.format(date1970) + "   " + tzBahrain.getID());
System.out.println();

Date date2016 = fmtDatetimeAlgiers.parse("2016-01-01 09:44:00");
System.out.println(fmtDatetimeAlgiers.format(date2016) + "               " + tzAlgiers.getID());
System.out.println(fmtFullAlgiers.format(date2016) + "   " + tzAlgiers.getID());
System.out.println(fmtFullBahrain.format(date2016) + "   " + tzBahrain.getID());
System.out.println();

Output

09:44:00                          Africa/Algiers
1970-01-01 09:44:00 CET (+0000)   Africa/Algiers
1970-01-01 13:44:00 AST (+0400)   Asia/Bahrain

2016-01-01 09:44:00               Africa/Algiers
2016-01-01 09:44:00 CET (+0100)   Africa/Algiers
2016-01-01 11:44:00 AST (+0300)   Asia/Bahrain

The timezone names CET and AST are actually wrong for 1970. Algiers was using GMT, and Bahrain was using GST (Gulf Standard Time).