My book contains the following two examples: 1) ένα παιδί τριών χρονών 2) μια αναβολή δύο χρόνων The first example seems to use χρόνια, whereas the second one χρόνος. Could these two be swapped and still make sense here? And then for another example it has κατά τη μακρά περίοδο τριάντα ετών. Could one of the forms above be used here too? My book doesn't distinguish any nuances in meaning.