date & time formatting & de/serialization with Gson and ThreeTenABP

build.gradle

dependencies {
   implementation "com.google.code.gson:gson:2.8.6"
   implementation "com.jakewharton.threetenabp:threetenabp:1.2.2"
}

code.kt

val dateTime: OffsetDateTime //this comes from the backend in format "2016-10-26T12:00:00-06:00"
val stringDate = DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM).format(dateTime) //shows the date in locally adapted format

GsonSetup.kt

private fun createGson(): Gson = GsonBuilder()
    .registerTypeAdapter(OffsetDateTime::class.java, OffsetDateTimeTypeAdapter())
    .create()

class OffsetDateTimeTypeAdapter : JsonSerializer<OffsetDateTime>, JsonDeserializer<OffsetDateTime> {

    override fun serialize(
        src: OffsetDateTime,
        typeOfSrc: Type,
        context: JsonSerializationContext
    ): JsonElement = JsonPrimitive(FORMATTER.format(src))

    @Throws(JsonParseException::class)
    override fun deserialize(
        json: JsonElement,
        typeOfT: Type,
        context: JsonDeserializationContext
    ): OffsetDateTime = FORMATTER.parse(json.asString, OffsetDateTime.FROM)

    companion object {
        private val FORMATTER = DateTimeFormatter.ISO_OFFSET_DATE_TIME //https://en.wikipedia.org/wiki/ISO_8601
    }
}