UdashFramework/udash-core

Hi @72squared, default serialization is indeed based on GenCodec (it's possible to integrate with something else if necessary, e.g. Circe). Using RestDataCompanion causes a GenCodec to be derived for your type. Then, AsRawReal is inferred based on this GenCodec (see this diagram for more details on how these implicits interact).
However, when we're already at the case class level then we're working with just GenCodec (no AsRawReal here). Therefore, you need an instance of GenCodec[DateTime]. The best way to do this now is to create an alternative implementation of RestDataCompanion that will inject additional implicits:

abstract class EnhancedRestDataCompanion[T](implicit
  instances: MacroInstances[EnhancedRestImplicits, CodecWithStructure[T]]
) extends {
  implicit lazy val codec: GenCodec[T] = instances(EnhancedRestImplicits, this).codec
  implicit lazy val restStructure: RestStructure[T] = instances(EnhancedRestImplicits, this).structure
  implicit lazy val restSchema: RestSchema[T] = RestSchema.lazySchema(restStructure.standaloneSchema)
}

The details of the above implementation don't matter - is is almost exactly the same as RestDataCompanion except that it injects more implicits into typeclass materialization.
Then, you put a GenCodec[DateTime] into EnhancedRestImplicits:

trait EnhancedRestImplicits extends DefaultRestImplicits {
  implicit val dateTimeCodec: GenCodec[DateTime] = GenCodec.nullableSimple(i => DateTime.parse(i.readString()), (o, v) => o.writeString(v.toString))
}

Note that you no longer need an instance of AsRawReal[JsonValue, DateTime] because it will be inferred from this GenCodec.
Now you can write:

case class User(name: String, createdAt: DateTime)
object User extends EnhancedRestDataCompanion[User]

Also, I highly discourage you from using DateTime.now() (or any other impure function) as a default parameter value.