The answer
It does not mean a literal twenty-four-hour time period, but an age of time that may extend for millions of years. The sun, the moon, and the planets were not created until the fourth age. Therefore, before their creation, there could have been no measurement of time as we experience it today, nor could the earth have experienced day and night in any physical sense. This was affirmed by many Church Fathers, such as St. Jerome.1143
Fr. Tadros Yacoub Malaty
Notes
1143 St. Jerome, Hom. 1.