in json/src/main/scala/com/gu/contentapi/json/CirceEncoders.scala [190:194]
def genDateTimeEncoder(truncate: Boolean = true): Encoder[CapiDateTime] = Encoder.instance[CapiDateTime] { capiDateTime =>
val dateTime: OffsetDateTime = OffsetDateTime.parse(capiDateTime.iso8601)
// We don't include millis in JSON, for backwards-compatibility
Json.fromString(DateTimeFormatter.ISO_OFFSET_DATE_TIME.format(if (truncate) dateTime.truncatedTo(ChronoUnit.SECONDS) else dateTime))
}