Which years were prime?

1973 was the only one.
1967 = 7 x 281
1968 = 2 x 2 x 2 x 2 x 3 x 41
1969 = 11 x 179
1970 = 2 x 5 x 197
1971 = 3 x 3 x 3 x 73
1972 = 2 x 2 x 17 x 29
1973 PRIME
1974 = 2 x 3 x 7 x 47