Java Date/Calendar oddness

Question

I have a bit of (Java) that I where I am trying to simply subtract 7 days from the current date. It seemed to me like Calendar.add(..) should be the method to use (and what previous questions here seem to say), so that's what I tried:

SimpleDateFormat df = new SimpleDateFormat("dd-mm-yyyy");
GregorianCalendar cal = (GregorianCalendar) GregorianCalendar.getInstance();
System.out.println("ReportUtil.getDefaultReportStartDate cal: "+cal.toString() );
System.out.println("PRE ReportUtil.getDefaultReportStartDate: "+df.format(cal.getTime()) );
cal.add(Calendar.DATE, -7);
System.out.println("POST ReportUtil.getDefaultReportStartDate: "+df.format(cal.getTime()) );

That looks ok to me but you'll see from the output below the month field seems to go a bit... sideways! The day of the month/date seems to change correctly, but what is going on with the month?!

ReportUtil.getDefaultReportStartDate cal: java.util.GregorianCalendar[time=1330098699960,areFieldsSet=true,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="GB-Eire",offset=0,dstSavings=3600000,useDaylight=true,transitions=242,lastRule=java.util.SimpleTimeZone[id=GB-Eire,offset=0,dstSavings=3600000,useDaylight=true,startYear=0,startMode=2,startMonth=2,startDay=-1,startDayOfWeek=1,startTime=3600000,startTimeMode=2,endMode=2,endMonth=9,endDay=-1,endDayOfWeek=1,endTime=3600000,endTimeMode=2]],firstDayOfWeek=2,minimalDaysInFirstWeek=4,ERA=1,YEAR=2012,MONTH=1,WEEK_OF_YEAR=8,WEEK_OF_MONTH=4,DAY_OF_MONTH=24,DAY_OF_YEAR=55,DAY_OF_WEEK=6,DAY_OF_WEEK_IN_MONTH=4,AM_PM=1,HOUR=3,HOUR_OF_DAY=15,MINUTE=51,SECOND=39,MILLISECOND=960,ZONE_OFFSET=0,DST_OFFSET=0]

PRE ReportUtil.getDefaultReportStartDate: 24-51-2012
POST ReportUtil.getDefaultReportStartDate: 17-51-2012

Answer 1

SimpleDateFormat df = new SimpleDateFormat("dd-mm-yyyy");

You get a strange month value because mm means minutes. Try:

 SimpleDateFormat df = new SimpleDateFormat("dd-MM-yyyy");

You can consult the whole list of the format symbols here: http://docs.oracle.com/javase/6/docs/api/java/text/SimpleDateFormat.html

Answer 2

mm is the format string for Minute. You want MM

Answer 3

Your result seems to be correct.

The month is "1" in both dates of your first log line, which means February.

The "-mm-" in your SimpleDateFormat means minute and not month, thus the odd month of "51"