Wonky dating in Java

Question

The following Java code just parses a date (with time portion) 2009-01-28-09:11:12 using SimpleDateFormat. Let's have look at it.

final public class Main
{
    public static void main(String[] args)
    {            
        try
        {
            DateFormat df = new SimpleDateFormat("yyyyMMddHHmmss");
            Date d = df.parse("2009-01-28-09:11:12");
            System.out.println(d);
        }
        catch (ParseException ex)
        {
            Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

The date (with time) displayed (after parsing) by the above code is as follows,

Sun Nov 30 22:07:51 IST 2008

even though we are attempting to parse the date 2009-01-28-09:11:12. It looks somewhat wonky. Why does it parse so?

Answer 1

Shouldn't your date format be something like this:

DateFormat df = new SimpleDateFormat("yyyy-MM-dd-HH:mm:ss");

to match this format:

Date d = df.parse("2009-01-28-09:11:12");

?

As for why, per this:

• http://docs.oracle.com/javase/6/docs/api/java/text/SimpleDateFormat.html

the parser actually looks at these as numbers, and the trick is that - is a part of a number, representing negative numbers. So if you do:

df.parse("2009-01-02-00:00:00")

it gives:

Mon Dec 01 00:02:00 EST 2008

That parses 2009 as yyyy, then -0 as MM (which is previous month as months start from 1), then 1 as dd, etc.

As per parse in DateFormat:

• http://docs.oracle.com/javase/6/docs/api/java/text/DateFormat.html#parse(java.lang.String)

By default, parsing is lenient: If the input is not in the form used by this object's format method but can still be parsed as a date, then the parse succeeds. Clients may insist on strict adherence to the format by calling setLenient(false).

I guess, if you have an option, it would be better to use slashes instead of dashes, if you like formats like 2009/01/02 12:34:56. This:

df.parse("2009/01/02-00:00:00")

will throw an exception:

ERROR java.text.ParseException:
Unparseable date: "2009/01/02-00:00:00"

I can only conclude it's a very good thing that / is not considered a number division by DateFormat...

Answer 2

Well, you could define the format that you actually are parsing...

Answer 3

The dates are parsed in lenient mode by default. This means that it's forgiving to errors like parsing feb 29 as mar 1 if it's not a leap year. What you are asking it to parse is the -28th day of the -1st month of 2009, and it dutifully tries to give you exactly what you ask for.

To turn off this behaviour call format.setLenient(false). Then the dateformat will throw an exception if you try to parse something that's not a real date.