perhaps this response is a bit late, but I happened to stumble across your blog entry through Google. I would fully agree with Jan-Kees: use Joda Time. Some reasons in addition to those mentioned by Jan-Kees:

- Joda Time provides for a LocalDate type (pure date, no time, no timezone). This is often exactly what you need to register something like a date of birth. Using java.util.{Date,Calendar}, you’re forced to use an absolute millisecond-since-epoch to represent a date. However, extracting the date part out of a millisecond-since-epoch is time zone dependent. This may cause all kinds of subtle bugs.
- The Joda Time API is much more suitable for use in a multi-threaded environment, because its value types are immutable and the formatters are thread safe.

There are many more advantages, but I guess these are the most important ones for me.

As for JDK7 and JSR-310: I think it might still be years before JDK7 that’s going to be widely used, I’m certainly not going to wait for that. I hope JSR-310 will become available as well for JDK5/6, but I’m not sure what the status is.

I came across your blog because I’m currently working on a project to get Joda-Time working with Toplink. Currently, JPA/Hibernate support is available, but not JPA/Toplink Essentials and JPA/EclipseLink. I’m gonna post a message on my blog when it’s ready.

Kind regards,
Frans van Buul

Second, because it doesn’t have the strange behavior you describe. Check:

Calendar c = Calendar.getInstance();
c.set(Calendar.MONTH, Calendar.JANUARY);
c.set(Calendar.DAY_OF_MONTH, 31);
c.set(Calendar.YEAR, 2010);

// Construct 31-01-2010 date
DateTime dateTime = new DateTime(c);

// Set month to February
dateTime = dateTime.withMonthOfYear(DateTimeConstants.FEBRUARY);

// Get the max value
int maximumValue = dateTime.dayOfMonth().getMaximumValue();
System.out.println("Joda maximumValue = " + maximumValue);

————–
Prints:
Joda maximumValue = 28