Upvote:3
The Catholic Church's defense, if there is any that is required, comes from the Sacred Deposit of faith = Sacred Scripture + Holy Tradition.
One would look at what the Church has taught, consistent with the deposit of faith, throughout the ages.
One such teaching is from Pope Leo XIII in Providentissimus Deus the Encyclical on the Study of Holy Scripture given in 1893 (please note Charles Darwin's book On the Origin of Species came out in 1859). In No. 18 under The Authority of Holy Scripture; Modern Criticism; Physical Science, the Pope quoting St. Augustine, a Church father and St. Thomas Aquinas, the Angelic Doctor, teaches
18 [...] There can never, indeed, be any real discrepancy between the theologian and the physicist, as long as each confines himself within his own lines, and both are careful, as St. Augustine warns us, "not to make rash assertions, or to assert what is not known as known."1 If dissension should arise between them, here is the rule also laid down by St. Augustine, for the theologian: "Whatever they can really demonstrate to be true of physical nature, we must show to be capable of reconciliation with our Scriptures; and whatever they assert in their treatises which is contrary to these Scriptures of ours, that is to Catholic faith, we must either prove it as well as we can to be entirely false, or at all events we must, without the smallest hesitation, believe it to be so."2 To understand how just is the rule here formulated we must remember, first, that the sacred writers, or to speak more accurately, the Holy Ghost "Who spoke by them, did not intend to teach men these things (that is to say, the essential nature of the things of the visible universe), things in no way profitable unto salvation."3 Hence they did not seek to penetrate the secrets of nature, but rather described and dealt with things in more or less figurative language, or in terms which were commonly used at the time, and which in many instances are in daily use at this day, even by the most eminent men of science. Ordinary speech primarily and properly describes what comes under the senses; and somewhat in the same way the sacred writers-as the Angelic Doctor also reminds us - `went by what sensibly appeared,"4 or put down what God, speaking to men, signified, in the way men could understand and were accustomed to.
1. In Gen. op. imperf. ix., 30.
2. De Gen. ad litt. i. 21, 41.
3. S. Aug. ib. ii., 9, 20.
4. Summa theol. p. I, q. lxx., a. I, ad 3.
Thus
Addendum
From The Replies of the Pontifical Biblical Commission On questions of Sacred Scripture Translated by E. F. Sutcliffe, S.J. (a translation of this) point No.
VIII: In the designation and distinction of the six days mentioned in the first chapter of Genesis may the word Yom (day) be taken either in the literal sense for the natural day or in an applied sense for a certain space of time, and may this question be the subject of free discussion among exegetes?
Answer: In the affirmative.
Answers OP: In the Catholic Church one is free to understand "day" in Genesis literally, or as a certain space of time.
Reader is encouraged to read all the points I through VIII.