It isn't a common or official belief. H.H. Pope Shenouda III argues strongly that the days refer to ages.
Question: How can the saying of the Bible that God created the world in six days coincide with the opinion of the geolgists that the age of the earth is thousands even millions of years?.
answer: The days of creation are not Solard ays as our days now... The day of creation is a period of time, not known how long, which could have been a second or thousands or millions of years. This period was determined by the saying "so the evening and the morning were..."
The evidences for this are many, among which are:
1. The Solar day is the period of time between the sunrise and its rising again or between the sunset and its settinga gagin. Since the Sun was only created on the fourth day (Gn 1:16-19)... then the first four days were not solar days.
2. As for the seventh day, the Bible did not state that it has ended... The Bible did not say [so the evening and the morning were the seventh day[, and thousands of years passed from Adam till now while this seventhe day is still going on. Accordingly, the days of creation are not Solar days but unkown periods of time.
3. As a whole, the Bible said about the creation and its six days: "This is the history of the heavens and the earth when they were reated, in (the day) that the Lord God made the earth and the heavens." (Gn 2:4). So the Bible summed up in the word (day) all the six days of creation...
Let the geologists ay then whatever they want about the age fo the earth; for the Bible did not mention any age fo rthe earth that may contradict the view of the geologists. The way the Lord looks to the measure of time is explained by the apostle as follows: "With the Lord one day is as a thousand years, and a thousand years are like one day" (2 Pet 3:
Since the book you posted is rather long, could you point to the page which supports young earth beliefs? Glancing at it quickly it seems to be saying about the same thing.