Caesium atomic clocks are extraordinarily accurate; if a clock had been made at the time of the dinosaurs, it would be off by less than four seconds (after 80 million years).
sˈiːsiəm ɐtˈɑːmɪk klˈɑːks ɑːɹ ɛkstɹˌoːɹdɪnˈɛɹᵻli ˈækjʊɹət; ɪf ɐ klˈɑːk hɐdbɪn mˌeɪd æt ðə tˈaɪm ʌvðə dˈaɪnəsˌɔːɹz, ɪt wʊd biː ˈɔf baɪ lˈɛs ðɐn fˈoːɹ sˈɛkəndz ˈæftɚɹ ˈeɪɾi mˈɪliən jˈɪɹz.