Calendar hour set to 0, yet displays 1, why?

Question

I am running into a strange issue with Java Calendar.

The following code adds 3 hours consecutively from a date starting at midnight.

public static void main(String[] args) {

    Calendar now = Calendar.getInstance(TimeZone.getTimeZone("GMT+0"));

    // Setting to January 29th, 1920 at 00:00:00
    // now.setTimeZone(TimeZone.getTimeZone("GMT+0"));
    now.set(Calendar.YEAR, 1920);
    now.set(Calendar.MONTH, 0);
    now.set(Calendar.DAY_OF_MONTH, 29);
    now.set(Calendar.HOUR_OF_DAY, 0);
    now.set(Calendar.MINUTE, 0);
    now.set(Calendar.SECOND, 0);
    now.set(Calendar.MILLISECOND, 0);       

    now.setLenient(false);

    int threeHours = 1000 * 60 * 60 * 3;

    SimpleDateFormat sdf
        = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss SSS");

    for (int i=0;i<25;i++) {

        System.out.println(sdf.format(now.getTime()));
        now.add(Calendar.MILLISECOND, threeHours);

    }

}

Yet, the displayed result is:

1920-01-29 01:00:00 000
1920-01-29 04:00:00 000
1920-01-29 07:00:00 000
1920-01-29 10:00:00 000
1920-01-29 13:00:00 000
1920-01-29 16:00:00 000
1920-01-29 19:00:00 000
1920-01-29 22:00:00 000
1920-01-30 01:00:00 000
1920-01-30 04:00:00 000
1920-01-30 07:00:00 000
1920-01-30 10:00:00 000
1920-01-30 13:00:00 000
1920-01-30 16:00:00 000
1920-01-30 19:00:00 000
1920-01-30 22:00:00 000
1920-01-31 01:00:00 000
1920-01-31 04:00:00 000
1920-01-31 07:00:00 000
1920-01-31 10:00:00 000
1920-01-31 13:00:00 000
1920-01-31 16:00:00 000
1920-01-31 19:00:00 000
1920-01-31 22:00:00 000
1920-02-01 01:00:00 000

Why is the first hour 1 and not 0? I am located at GMT+1, could this be related?

Answer 1

Using now.getTime() gets the Date which doesn't have a time zone.

Try setting the timezone with

sdf.setTimeZone(now.getTimeZone());

Answer 2

i guess you should use now.set(Calendar.HOUR, 0); instead now.set(Calendar.HOUR_OF_DAY, 0);

Answer 3

now it should work fine

        Calendar now = Calendar.getInstance();
        TimeZone timezone = TimeZone.getTimeZone("GMT+0");
        now.set(Calendar.YEAR, 1920);
        now.set(Calendar.MONTH, 0);
        now.set(Calendar.DAY_OF_MONTH, 29);
        now.set(Calendar.HOUR_OF_DAY, 0);
        now.set(Calendar.MINUTE, 0);
        now.set(Calendar.SECOND, 0);
        now.set(Calendar.MILLISECOND, 0);       
        now.setLenient(false);
        int threeHours = 1000 * 60 * 60 * 3;
        SimpleDateFormat sdf
            = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss SSS");
        sdf.setTimeZone(timezone);
        for (int i=0;i<25;i++) {
            System.out.println(sdf.format(now.getTime()));
            now.add(Calendar.MILLISECOND, threeHours);
        }