What is the best way to parse a date in MM/DD/YY format and adjust it to the current / previous century?

Question

One of our customers wants to be able to enter a date with only 2 digits for the year component. The date will be in the past, so we want it to work for the previous century if the 2 digit year is after the current year, but work for the current century if the 2 digit year is equal to or less than the current year.

as of today 10/30/2008

01/01/01 = 01/01/2001

01/01/09 = 01/01/1909

This is a strange requirement, and I solved the problem, I just don't like my solution. It feels like there is a better way to do this.

Thanks for the help.

public static String stupidDate(String dateString)
{
 String twoDigitYear = StringUtils.right(dateString, 2);
 String newDate = StringUtils.left(dateString, dateString.length() - 2);
 int year = NumberUtils.toInt(twoDigitYear);
 Calendar c = GregorianCalendar.getInstance();
 int centuryInt = c.get(Calendar.YEAR) - year;
 newDate = newDate + StringUtils.left(Integer.toString(centuryInt), 2) + twoDigitYear;
 return newDate;
}

Answer 1

It's been a while since I've used java, so I'm going to use pseudo code.

If I understand what you're trying to get at, the easiest solution is something like this:

myDate = ParseDate(thatString);
if (myDate > Now)
    myDate = myDate.AddYears(-100)

Answer 2

How about this:

public static String anEasierStupidDateWithNoStringParsing(String dateString) {
    DateFormat df = new SimpleDateFormat("MM/dd/yyyy");

    //handling ParseExceptions is an exercise left to the reader!
    Date date = df.parse(dateString);
    Calendar cal = Calendar.getInstance();
    cal.setTime(date);

    Calendar now = Calendar.getInstance();
    if (cal.after(now)) {
        cal.add(Calendar.YEAR, -100);
    }

    return cal;
}

In other words, let SimpleDateFormat parse the String and just adjust the year to be the previous century if SimpleDateFormat (which has it's own rules for interpreting year strings) returns a date that is after the current date.

This would guarantee that all dates returned are in the past. However, it doesn't account for any dates that might be parsed as before this past century - for example, with the format MM/dd/yyyy, a date string like "01/11/12" parses to Jan 11, 12 A.D.

Answer 3

If Joda Time is an option:

String inputDate = "01/01/08";
// assuming U.S. style date, since it's not clear from your original question
DateTimeFormatter parser = DateTimeFormat.forPattern("MM/dd/yy");
DateTime dateTime = parser.parseDateTime(inputDate);
// if after current time
if (dateTime.isAfter(new DateTime())) {
    dateTime = dateTime.minus(Years.ONE);
}

return dateTime.toString("MM/dd/yyyy");

I know Joda Time isn't part of Java SE, and as I've said in another thread, I usually do not condone using a third-party library when there's a Java library that does the same thing. However, the person who is developing Joda Time is also leading JSR310 - the Date and Time API that'll make it into Java 7. So I Joda Time is basically going to be in future Java releases.

Answer 4

SimpleDateFormat already does two-digit year parsing for you, using the two-letter ‘yy’ format. (It'll still allow four digits, obviously.)

By default it uses now-80→now+20, so it's not exactly the same rule you propose, but it's reasonable and standardised (in the Java world at least), and can be overridden using set2DigitYearStart() if you want.

DateFormat informat= new SimpleDateFormat("MM/dd/yy");
DateFormat outformat= new SimpleDateFormat("MM/dd/yyyy");
return outformat.format(informat.parse(dateString));

In the longer term, try to migrate to ISO8601 date formatting (yyyy-MM-dd), because MM/dd/yy is approximately the worst possible date format and is bound to cause problems eventually.

Answer 5

Groovy script (easy enough to throw into java) demonstrating the point @bobince made about SimpleDateFormat.

java.text.SimpleDateFormat sdf = new java.text.SimpleDateFormat('MM/dd/yy')
java.text.SimpleDateFormat fmt = new java.text.SimpleDateFormat('yyyy-MM-dd')
Calendar cal = Calendar.getInstance()
cal.add(Calendar.YEAR, -100)
sdf.set2DigitYearStart(cal.getTime())
dates = ['01/01/01', '10/30/08','01/01/09']
dates.each {String d ->
  println fmt.format(sdf.parse(d))
}

Yeilds

2001-01-01
2008-10-30
1909-01-01

Answer 6

The best way is to use the Joda Time API for dates and times in Java.