How to define an Ordering in Scala?

object Joda {
    implicit def dateTimeOrdering: Ordering[DateTime] = Ordering.fromLessThan(_ isBefore _)
}

// elsewhere
import Joda._
dateTimes.sorted

To facilitate working with Joda DateTime in Scala, nscala-time was created: https://github.com/nscala-time/nscala-time

After including it in your project with

libraryDependencies += "com.github.nscala-time" %% "nscala-time" % "1.8.0"

you can just import OrderingImplicits. Either all at once:

import com.github.nscala_time.time.OrderingImplicits._

or a particular one:

import com.github.nscala_time.time.OrderingImplicits.DateTimeOrdering