datetime - Why does an hour get added on to java.util.Date for dates before Nov 1 1971?

Question

The following code appears to demonstrate a bug in java.util.Date whereby an hour gets added on if the local clock is set to GMT with DST adjustment on and the time is before Nov 1 1971. My first assumption is always that I've got it wrong. Can anyone see what's wrong (or is this really a Java bug)? What's significant about Nov 1 1971?

import java.text.SimpleDateFormat;
import java.util.Locale;
import java.util.TimeZone;

class JavaUtilDateBug
{
    private static void demo() throws Exception
    {
        // UK developers usually have the clock on their development machines set
        // to "Europe/London" (i.e. GMT with daylight saving). Set it explicitly 
        // here so readers in other countries can see the problem too.
        TimeZone.setDefault(TimeZone.getTimeZone("Europe/London"));
        Locale.setDefault(Locale.ENGLISH);

        SimpleDateFormat dateFormat = new SimpleDateFormat("EEE MMM dd HH:mm:ss z yyyy");
        String strJan1st1970Expected = "Thu Jan 01 00:00:00 GMT 1970";
        String strJan1st1970Actual = dateFormat.parse(strJan1st1970Expected).toString();
        System.out.println("strJan1st1970Actual: " + strJan1st1970Actual); // -> "Thu Jan 01 01:00:00 GMT 1970"
        boolean jvmHasDateBug = !strJan1st1970Expected.equals(strJan1st1970Actual);
        System.out.println("jvmHasDateBug: " + jvmHasDateBug); // -> true

        // The anomaly only seems to affect times before 1 Nov 1971.
        final String strNov1st1971 = "Mon Nov 01 00:00:00 GMT 1971";
        assert strNov1st1971.equals(dateFormat.parse(strNov1st1971).toString());
    }

    public static void main(String[] args)
    {
        try
        {
            demo();
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }
}

My Java environment:

  java version "1.6.0_13"
  Java(TM) SE Runtime Environment (build 1.6.0_13-b03)
  Java HotSpot(TM) Client VM (build 11.3-b02, mixed mode, sharing)

Answer 1

There was a trial of British Standard Time between 27th Oct 1968 and 31st Oct 1971, which I suspect is what's causing this issue.

There's some details of the trial here:

http://en.wikipedia.org/wiki/British_Summer_Time#Single.2FDouble_Summer_Time

The timezone for Europe/London in Jan 1st 1970 was British Standard Time (GMT+1) so when you use a java.text.SimpleDateFormat to parse Jan 01 00:00:00 GMT 1970 it generates the correct epoch value equal to Jan 01 01:00:00 1970 in BST.

Then, due to the crappiness of java.util.Date, when you call java.util.Date.toString() it uses the default timezone for the current local as of now, which has changed to GMT and you get Jan 01 01:00:00 GMT 1970.