When my mind’s eye reads the phrase “period of time” I change nothing. I may even ask one to look at the word “DAY”.
When we say “Day” do we mean 24 hours?
When we say “Day” do we mean those hours of DAYlight as opposed to those hours of darkness called night?
Day and Day..one word with two separate meanings, sometimes in the same sentence.
Word meanings DO change over time. Example in 2009
year 1959 Gay is someone who is a good happy soul.
year 2009 Gay is something completely different!!
Technically, a “day” IS a “period of time”
Sure, in a general discussion about day and night you are correct. The word can take on various meanings.
However, if you confine yourself to the passage being discussed, Genesis 1, in order to discuss these particular events you will find that ver. 5 defines the first day as that in which the separation of light and darkness occurred, implying a span of 24 hours since it does not mention a series of separations of light and darkness...just one.
Thereafter in the passage the same standard can be applied since ...ex: ver. 8..."and there was evening and there was morning the second day" which points in the same way to another 24 hour period.
One can attempt to read between the lines and look at different ways "day" is used in other cultures and at other times, but to look simply and "scientifically" at the evidence before us here it seems the passage is fairly clear.