Search code examples
javacalendargregorian-calendar

java GregorianCalendar adding hour strange behavior

I have a problem with GregorianCalendar so if you please can help me out with it. First I'll give you my code:

private String changeClock(String day, String clock, int change) {
    String time="";
    DateFormat df=new SimpleDateFormat("yyyy-MM-dd hh:mm");
    try {
        Date d=df.parse(day+" "+clock);
        GregorianCalendar g=new GregorianCalendar();
        g.setTime(d);
        g.add(GregorianCalendar.HOUR_OF_DAY, change);
        time=g.get(GregorianCalendar.YEAR)+"-"
                +(g.get(GregorianCalendar.MONTH)+1)+"-"
                +g.get(GregorianCalendar.DAY_OF_MONTH)+" "
                +g.get(GregorianCalendar.HOUR_OF_DAY)+":"
                +g.get(GregorianCalendar.MINUTE);
    } catch (Exception e) {
        e.printStackTrace();
    }
    return time;
}

Let me explain what is happening. I have a GUI with + and - button. When someone press + it add one hour, or if - is pressed then take one hour.

Now example, time is 23:00 and I press +, it is everything ok and it jumps to 00:00 of the next day. Problems are on 12:00. If it is 12:00 and I press + it goes to 1:00 and that goes on and on. It doesn't move to the next day even after 2x12 hours or 21465x12 hours. Moving backward is a little better if I can say so. When it is 00:00 and I press - it changes to yesterday 23:00 (also date changes). If I then press + it changes also one day forward (so to today in this case).

What have I done wrong or what more should I write to my code?

Thanks for your help guys.

Solution

  • Your date format is wrong...

    You're using hh, which is a representation of the "Hour in am/pm (1-12)", so a time of 1pm is been converted to 1am instead.

    You should be using HH which is a a representation of the "Hour in day (0-23)".

    Either that, or you need supply a date/time format with the am/pm marker...

    Using either DateFormat df=new SimpleDateFormat("yyyy-MM-dd HH:mm"); or DateFormat df=new SimpleDateFormat("yyyy-MM-dd hh:mm a");

    Instead of relying on String date/time values, you should be passing in and back a Date object, leave the formatting for the display.