At the risk of appearing low-tech, a much more simple, Goroutine-safe solution for dealing with "now-dependent" code:

type NowFunc func() time.Time

func getGreeting(nowFunc NowFunc) string {

  now := nowFunc()

  if now.Hour() < 12 {

    return "Good Morning"

  }

  return "Good day"

}

And just pass in `time.Now` in for live code, and your own inline function for simulating a time in tests.