use of diff prepositions and meaning intended by "colloquially"

Question

—from lesson
To express in those days in French, you won't colloquially use ces jours-là, but rather à cette époque-là or en ce temps-là.
 
À cette époque-là, les femmes n'avaient pas le droit de vote.  
In those days, women didn't have the right to vote.
—
 
In the first sentence, which I copy from the lesson, what is “colloquially” meant to convey? That “ces jours-là” is wrong but it’s used anyway?  Or?
 
When a preposition is used, are there particular rules for using à vs. en vs.  dans?   Would “en cette époque-là” be incorrect?  Likewise for “en ce temps-là” ?  Or “dans ce temps-là”?  
 
I came across this sentence recently:
Dans ce temps-là, ajoute Gaétan, il n’y avait pas de lampe électrique. Juste des bougies. Brrrrrr ! 
 
It is from Le trésor du vieux moulin p. 101.  
https://beq.ebooksgratuits.com/contemporain/Boucher-moulin.pdf
 
Hence, my questions.  THX

Accepted Answer

Bonjour Melody,
Christian is correct that "colloquially" means in "informal situations".
Using "ces jours-là" in French is correct if you are specific about something:
Le mercredi et le vendredi, je m'occupe de moi. Donc, ces jours-là, je vais à la gym. = On Wednesdays and Fridays, I take care of myself. So, those days, I go to the gym.
-> "ces jours-là" refers to specific days (as example, days when you're free to do what you want)
You can also come across "ces jours-ci" to mean "at the moment".
Ces jours-ci, je vais à la gym le mercredi et le vendredi =These days (at the moment), I go to the gym on Wednesdays and Fridays
However, you would not say :

“en cette époque-là” but "à cette époque-là" / (you might also hear "à cette époque" - it is best to use the former expression as it makes it clearer)
“dans ce temps-là” but  "en ce temps-là"
“en ces jours-là” but "ces jours-là"

I hope this is helpful.
Bonne journée !

Answer

"colloquially" means what it normally it does: You would not use "ces jours-là" in informal situations, talking with your friends, colleagues, etc. As far as I can tell you can use it in formal writing to express "in those days" e.g. https://www.academie-francaise.fr/hommage-prononce-en-seance-loccasion-du-deces-de-maurice-druon-secretaire-perpetuel-honoraire:
"Il paraît en effet impossible de ne pas mettre en rapport l’insistance de d’Astier de La Vigerie, ces jours-là, d’écrire un chant pour la Résistance [...]"

I think for these phrases you just use the exact specific prepositions. As you noticed "dans ce temps-là" is used but from what I can tell only rarely. So I would stick to "en ce temps-là".

For the use of en vs dans as temporal prepositions in general, see https://kwiziq.learnfrenchwithalexa.com/revision/grammar/talking-about-time-when-to-use-en-versus-dans-prepositions-of-time. But I guess you know this already.

Answer

Céline, thank you very much for your helpful reply.