How to parse four digit year only (with Joda Time)?

Question

Is there a way to force Joda time to parse dates only when they contain four digit years? For example:

• 2009-11-11 - should parse
• 09-11-11 - should not parse

Tried the following code:

DateTimeFormatterBuilder builder = new DateTimeFormatterBuilder();
DateTimeFormatter formatter = builder.appendYear(4, 4).appendLiteral('-').appendMonthOfYear(1).appendLiteral('-').appendDayOfMonth(1).toFormatter();
formatter.parseDateTime("09-11-11");

Parses into 0009-11-11. Apparently minDigits in the method appendYear are only used for formatting when printing out the date.
The result is the same if I use appendYearOfEra(). If I use appendYearOfCentury(), it parses the year into 1909 instead.

We are implementing a general data parser, which will recognize various types of inputs. Also the example is a shortened form of the real deal (for simplicity). Real life scenarios parses dates which can have weekdays, months as words, time, zone and different characters separating month, day and year. Therefore, writing a RegEx or checking the content/length of the string can prove rather difficult.

Some real examples could look like this:

• 2009-11-11
• Wednesday 2009-11-11T15:00:00
• 2009/11/11 15:00
• and many more...

Answer 1

You can build extremely specific parsers and formatters using DateTimeFormatterBuilder. There's generally no need to use this class directly, since most common formats are more easily available elsewhere in the API, but this is the builder class they all use under the covers.

Answer 2

You can check the length of the date string.

Answer 3

What do you want to get from a user who enters '0001-01-01' as the date (that is, they entered 4 digits for the year, but the first three were zeroes)? What about '0999-12-31'? And '999-12-31'? And what about '10000-01-01' - the infamous Y10K¹ problem?

• If that is a legitimate value, then you are stuck with discovering the length of what the user typed as the year portion of the date (probably after any other parsing has been done), and making sure it is at least (or is it exactly?) four digits.

• If that is not a legitimate value, then you are stuck with checking the year value after the date is parsed.

• Or you can take the code and modify it so it includes your preferred definition of valid year.

---

¹ I do not plan to start working on fixing the Y10K problem before 5000-01-02.

Answer 4

DateTimeFormatterBuilder.appendFixedDecimal() may well do what you need.

Alternatively, you could implement the DateTimeParser interface to create whatever parser you want and pass that into the DateTimeFormatterBuilder.