Probably went with the simplest implementation, if starting from the current “seconds since epoch” value. Let the user do any calculations needed to translate three days into that measurement.

It also efficiently annoys the most people at once: those what want hours will complain if they set it to days, thought that want days will complain if hours are used. By using minutes or seconds you can wind up both segments while not offend those who rightly don't care because they can cope with a little arithmetic :)

Though doing what sleep(1) does would be my preference: default to seconds but allow m/h/d to be added to change that.