Two letter year, shall it be allowed?

Question

Recently after library updagrade of Apache POI, I upgraded some other API as well. The other library I used read all cell contents as String and then I had to parse this string into Date.

The problem occurred when user started entering date as dd-mm-yy, the year appeared as 00yy AD.

As per documentation of SimpleDateFormat

For parsing, if the number of pattern letters is more than 2, the year is interpreted literally, regardless of the number of digits. So using the pattern "MM/dd/yyyy", "01/11/12" parses to Jan 11, 12 A.D.

So the question is, is it a better to enter the four letter year over two letter year?

The another question is what is best way to predict the year if its in two letter format. Since the issue will come while parsing below year

Bond Start Date : 12-Jan-98 (1998)
Bond End Date   : 12-Jan-70 (2070)

Regards, Hanumant

Answer 1

It is not clear what you are asking.

1. If you are asking how to specify a date format that accepts 2 digit years (only) and interprets them conventionally, then you should use "dd-mm-yy".

2. If you are asking how to specify a date format that accepts 2 digit years and interprets them conventionally, AND ALSO handles 4 (or more) digit years, then you can't. As the javadoc says, if you use "dd-mm-yyyy", 2 digit years are interpreted as years in the first century AD.

   One possible solution is to use TWO formats. First attempt to parse using "dd-mm-yy", and if that fails, try "dd-mm-yyyy".

   But this is a hack ... and problematic if the user might actually need to enter a historical date.

3. If you are asking what you should do, then I'd recommend moving away from ambiguous ad-hoc formats that force you to (effectively) guess what the user means.

   • If the user has to enter dates / times in a character-based form, require them to use one of the ISO 8601 formats, and be strict when parsing the user-supplied date/time strings.

   • Otherwise, provide the user with a date picker widget.

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The another question is what is best way to predict the year if its in two letter format.

Well this is the nub of the problem isn't it! In the 20th century, we all knew what a 2 digit year meant. You just slapped 19 on the front. (Ah ... those were the good old days!)

Now it is necessary to use a different heuristic. And the heuristic that SimpleDateFormat uses is described by the javadoc thus:

"For parsing with the abbreviated year pattern ("y" or "yy"), SimpleDateFormat must interpret the abbreviated year relative to some century. It does this by adjusting dates to be within 80 years before and 20 years after the time the SimpleDateFormat instance is created. For example, using a pattern of "MM/dd/yy" and a SimpleDateFormat instance created on Jan 1, 1997, the string "01/11/12" would be interpreted as Jan 11, 2012 while the string "05/04/64" would be interpreted as May 4, 1964."

The heuristic is 80 years before to 20 years after "now". So actually 12-Jan-98 is in 1998 and 12-Jan-70 is in 1970 ... if you parse using a SimpleDateFormat with a "yy" format.

If you need the dates to mean something else, then you will need to use a different date parser. For example, if you use the Joda-time libraries, it is possible to specify the "pivot year"; i.e. the middle year of the century in which 2-digit years fall.

Reference:

• Joda-time Freaky Formatters