Extension.kt

import java.util.*
import kotlin.reflect.KMutableProperty1

fun <T> KMutableProperty1<T, Date>.addDays(receiver: T, nrOfDays: Int) {
    val cal = Calendar.getInstance()
    cal.time = this.get(receiver)
    cal.add(Calendar.DAY_OF_YEAR, nrOfDays)
    this.set(receiver, cal.time)
}

class C(var date: Date)

fun main(args: Array<String>) {
    val c = C(Date(0))
    C::date.addDays(c, 123)
    println(c.date)
}