Weird behaviour of jodatime in parsing some date formats

Question

I was trying to parse a date string using jodatime with a leading '+' before the yyyy part. I expected an error to be thrown, but it actually did not throw an error. I got outputs that don't make any sense instead:

System.out.println(DateTimeFormat.forPattern("yyyyMMdd").parseDateTime("20130101"));
// 2013-01-01T00:00:00.000+05:30 (Expected) (case 1)

System.out.println(DateTimeFormat.forPattern("yyyyMMdd").parseDateTime("+20130101"));
// 20130-10-01T00:00:00.000+05:30 (??? Notice that month changed to 10 also) (case 2)

System.out.println(DateTimeFormat.forPattern("MMyyyydd").parseDateTime("01+201301"));
// 20130-01-01T00:00:00.000+05:30 (??? At least month is fine this time) (case 3)

System.out.println(DateTimeFormat.forPattern("MM-yyyy-dd").parseDateTime("01-+2013-01"));
// 2013-01-01T00:00:00.000+05:30 (I expected an error, but this parsed correctly) (case 4)

Can anyone explain why this is happening? I expect either an exception, which means '+' sign is disallowed, or it should interpret +2013 as simply 2013, which is what it seems to do in the last case. But what is the deal with 20130 in case 2 and 3, and month = 10 in case 2?

Answer 1

If you want throw exception on + sign you can use DateTimeFormatterBuilder, it is more flexible.
E.g. for format yyyyMMdd

DateTimeFormatter dtf = new DateTimeFormatterBuilder()
         .appendFixedDecimal(DateTimeFieldType.year(), 4)
         .appendMonthOfYear(2)
         .appendDayOfMonth(2)
         .toFormatter();
dtf.parseDateTime("19990101");  - parsed correctly  
dtf.parseDateTime("-19990101"); - throw exception  
dtf.parseDateTime("+19990101"); - throw exception  

also this pattern already present in standart patterns:

ISODateTimeFormat::basicDate()

EDIT
But there is strange behaviour in case of using appendFixedSignedDecimal method:

DateTimeFormatter dtf = new DateTimeFormatterBuilder()
         .appendFixedSignedDecimal(DateTimeFieldType.year(), 4)
         .appendMonthOfYear(2)
         .appendDayOfMonth(2)
         .toFormatter();
dtf.parseDateTime("19990101");  - parsed correctly  
dtf.parseDateTime("-19990101"); - parsed correctly   (negative years)  
dtf.parseDateTime("+19990101"); - throw exception (???)

I think this is issue in Joda lib, because

DateTimeFormatter dtf = new DateTimeFormatterBuilder()
         .appendFixedSignedDecimal(DateTimeFieldType.year(), 4)
         .toFormatter();  

works as expected

dtf.parseDateTime("1999");  - parsed correctly  
dtf.parseDateTime("-1999"); - parsed correctly (negative years)  
dtf.parseDateTime("+1999"); - parsed correctly   

(This case is present in Unit Tests for joda library)

Answer 2

After going through joda-time code, I was able to narrow down the issue. It was being caused due to an anomalous increment in the code. I have opened an issue here. I also have a fix ready here. I'll raise a pull request once I get confirmation on is it is the right way to fix it.