"God has fixed a time that is right for everything to happen" or "God has set a time that is right for each thing to happen:
# placed eternity in their hearts
Here the word "their" refers to human beings. Here the "hearts" of the people represent their thoughts and desires. AT: "placed eternity in the hearts of human beings" or "caused people to think about eternal things" (See: [[rc://en/ta/man/translate/figs-metonymy]])