public void Add_DoesNotMutate()
{
var nowWatch = new DateTime(2001, 10, 10, 11, 05, 02);
TimeProvider.Now = () => nowWatch;
var watch = new Watch();
var nowFiveMinutesLater = new DateTime(2001, 10, 10, 11, 10, 59);
TimeProvider.Now = () => nowFiveMinutesLater;
var fiveMinutesLater = new Watch();
var newWatch = watch + fiveMinutesLater;
Assert.AreNotSame(newWatch, watch);
Assert.AreNotSame(newWatch, fiveMinutesLater);
Assert.AreEqual(nowWatch, watch.WhatTimeIsIt());
Assert.AreEqual(nowFiveMinutesLater, fiveMinutesLater.WhatTimeIsIt());
}