CHECK_CXX_SOURCE_RUNS("
    #include<time.h>
    int main(int, char *[]) {
      time_t t = -14210715; // 1969-07-20 12:34:45
      struct tm *ptm = gmtime(&t);
      return !(ptm && ptm->tm_year == 69);
    }"
  HAS_PRE_1970
)