I don't know if it's limited to JWs, dear Jam (peace to you!): there are a lot of folks out there who believe the physical earth (indeed, the physical universe) was created in seven literal 24-hour days. Unfortunately, that's an error.
I'm sure someone else has pointed it out here already (although I can't seem to find it) but that there were "seven" creative days, technically OR literally... is an error. Not even the Bible says the earth was created in seven days... whether it is speaking of seven 24-hour periods... or 7 eons of billions of years each.
What it does say is that there were only six creative days (again, regardless of what constitutes a "day"):
"By the seventh day,God had finished the work He had been doing and so He rested on the seventh day. And God blessed the seventh day and made it holy because on it He rested from all of the creative work He had been doing." Genesis 2:2, 3
[Yes, okay, as my Lord was recorded to say, the the Most Holy One of Israel is still working, but not with regard to creating anything physical. All of that was concluded on the sixth day.]
Following that, the part of the physical creation that most of us know as the planet Earth, wasn't, according to the Bible, created until the second and third days and was completed in two days (again, regardless of the actual timespan), not six OR seven days. Genesis 1:6-13
To address your comment as to "the period when the earth was formless and void," the "earth" spoken of at Genesis 1:1, is not the planet earth: the Hebrew word "'erets" is another example of the confused, if not false, stylus of the copyists: it can also mean "sheol" or... [the world of the] dead. Or... void of life, without life. Since the "light" had not yet come forth... life... had not yet come forth. This is another thread, though, so...
But I don't have to share with you that I received such from my Lord (I did): one only need (1) look up the word, in all of its renderings, and (2) read the verse in light of the subsequent verses. For example:
1. The "earth" was "formless" and "void". If that is true, the verse cannot be speaking of the planet earth, as the planet earth has a form: sphere(ical). If it was formLESS... then it had no shape... and no solidity. It certainly had no life or ability to support life (void). So, what is the "earth" here:
2. The physical realm (in contrast to the "heavens" or spirit realm)... before the creation of matter.
We can further know that this not referencing the planet earth because:
3. The "light" and "darkness" (also referred to as "day" and "night")... had absolutely nothing to do with sun and moon. Couldn't have. Why? Because those [luminaries] weren't created yet. They didn't come into existence until the fourth day. Genesis 1:13-19 So, the light/darkness... and day/night...mentioned at Genesis 1:1-5... is not that mentioned at Genesis 1:13-19.
My point, however, is that the belief/understanding/teaching, etc., that the earth was formed in seven days, be those days 24-hour periods each... or be they billions of years each... is not substantiated by the Genesis account and those who take the lead in teaching it really should go back and read it again.
OR... simply go to the Source and get correct information so at least they're teaching accurately.
Again, peace to you!
Your servant and a slave of Christ,
SA