From the introduction:
Time is ending.
That’s not the same as the world ending. It’s much worse. People — the people who pay attention — have always known that humanity has an end point. At some point in the future, we’ll have used up the planet’s resource or we’ll be struck by an asteroid or a zombie plague will wipe us out or we’ll nuke each other into oblivion. Something will end humanity’s time on this planet, and the planet will shake us off and get on with the business of spinning endlessly in outer space. So it goes.
But that’s not the same thing as time ending, and that’s what’s so scary. Time is ending in 2050. That’s when the time stream just... stops. Everything past that is dead calm, silence, Stillness. Someone past 2050 is watching, though. In the Stillness, someone is there. We can’t tell who or what they are, but someone in the Stillness sent back the first Message. So now, we have to get a message back to them, but we can’t just send it past 2050, because that’s when time stops.
I’m getting ahead of myself. Ha-ha! A little time traveler humor for you.