Basically, what you're asking is if we plant a time bomb, does it become morally neutral at a certain, sufficiently long time value, whereby it doesn't kill anyone who is alive today?

That time could be a lot shorter than 150 years. Say a one-year-long time bomb kills someone's newborn infant, and only that infant. When the bomb was planted, that infant hadn't yet been conceived. Of course that bomber is going behind bars.