Java, convert date to string and return the date that generated the error

I tried to convert date to string and then back to date But I find that the last date is different from the original date. What gives it?

//1975-06-20
    Calendar cal = Calendar.getInstance();
    cal.set(Calendar.YEAR,1975);
    cal.set(Calendar.MONTH,5);
    cal.set(Calendar.DAY_OF_MONTH,20);
    cal.set(Calendar.HOUR,0);
    cal.set(Calendar.MINUTE,0);
    cal.set(Calendar.SECOND,0);
    System.out.println(cal);

    Date originalDate = cal.getTime();
    System.out.println("Date 1: " + originalDate.toString());

    SimpleDateFormat sdf = new SimpleDateFormat("EEE MMM dd HH:mm:ss zzz yyyy");
    Date date = sdf.parse(originalDate.toString());
    System.out.println("Date 2: " + date.toString());

The output of the above code is:

cal: java.util.GregorianCalendar[time=?,areFieldsSet=false,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="Asia/Singapore",offset=28800000,dstSavings=0,useDaylight=false,transitions=9,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=1975,MONTH=5,WEEK_OF_YEAR=26,WEEK_OF_MONTH=5,DAY_OF_MONTH=20,DAY_OF_YEAR=179,DAY_OF_WEEK=4,DAY_OF_WEEK_IN_MONTH=4,AM_PM=1,HOUR=0,HOUR_OF_DAY=16,MINUTE=0,SECOND=0,MILLISECOND=333,ZONE_OFFSET=28800000,DST_OFFSET=0]
Date 1: Fri Jun 20 12:00:00 SGT 1975
Date 2: Fri Jun 20 11:30:00 SGT 1975

Solution

It may be due to the change in Singapore's time zone in 1982 (30 minutes)

@L_ 404_ 0@

When parsing the date, simpledateformat takes Sgt as UTC 8 and converts it to UTC 7.5, that is, Sgt before 1982. Therefore, the date is closed for 30 minutes