If it's off-topic here it should be closed.
If it's off-topic here and on-topic on the other site and of acceptable quality (we don't want to be the dumping ground for others - let's not make others our dumping ground) it should be closed and flagged for moderator attention. Ideally you would have experience with the other site to show that you know that the question would be on-topic and point to relevant parts in the tour/help center of the other site to make it easier for mods to see that you know what you are talking about.
If it's on-topic here, but would likely get better answers on another site you can leave a comment, again ideally with information from the tour/help center to show that the question would indeed be acceptable on the other site. You can then tell the user to decide themselves what they want to do and they can flag for moderator attention to migrate the question.
If it's on-topic and the person asking the question decides to not post the question somewhere else, even after getting a comment about the other site accepting such question, leave the question here.
It's the decision of the person asking the question where to post and if it's not off-topic there is no reason to close the question or migrate it against the will of the person asking the question, even if your opinion is that it would get infinitely better answers somewhere else. If it's on-topic here that's not your decision to make.
Give the person asking the question the benefit of doubt that they are indeed looking for worldbuilding context. Maybe they don't need exact dates and just want a rough idea. If so, why not give them a worldbuilding answer and say "Reality says roughly this, but judging by other things it could have also started at this other point in time".
It's absolutely fine and a nice thing to do to point someone somewhere else and say "You know, those guys are specialised towards this specific topic and you seem to be looking for real-world data. Maybe you should check out that site. According to the [help center] this question would be on-topic there. If you would like to migrate it you can flag your post for moderator attention and ask them to migrate it."
But please leave it at that if it's not off-topic. And while it's fine if multiple people think that a question can be migrated it's not useful for the person asking the question if the comment section is full of "Yeah, the other half a dozen guys are right - migrate this!" One or two comments, for example one comment like "You could ask to migrate there, but I am not so familiar with the site." and another one like "@XYZ is right, I am quite active on the site and we love such questions over there.", would be perfect. If there are already such comments you can simply upvote them.
We also don't want to make our site look worse than it really is. There are many experts here and while most people have a broad area of knowledge and are therefore not specific experts of each individual field you are likely to get at least a good starting point here and quite possibly high quality answers like those you would get on other sites. Pointing everyone somewhere else saying "Those guys are far better than us." is a bit harsh towards our own regular userbase. Of course, they specialise in a specific topic, so I understand the idea, but if a new user asks a question here and only ever gets "Go there, they are better" then this is starting to look quite bad.
Ideally you would just point out that other sites exist and might be useful to the OP for future questions. A short comment like "If you are interested in this topic check out our sister site XYZ.SE, they specialize on this topic and according to their [help center] they like this sort of question." The rest is up to the OP. This leaves it as just a friendly reminder that these other sites exist for the OP and future readers, while not undermining the authority of our own users and without making it look like it's off-topic.
A simple comment informing users that other Stacks exist is the same as pointing to any other site - it can be a nice and useful thing to do, but if you start insisting that people go somewhere else when in fact they are in the right place to get answers, even if there are other places with potentially more focused answers, things are getting difficult. Simple, short and friendly comments informing of the existence of other sites on the network are perfectly fine and should be written to help users, especially new ones - commands to migrate on-topic questions should not.
Some good examples for when a question is on-topic on WorldBuilding and you want to point out that a dedicated Stack exists:
If you are interested in constructed languages you might want to check out our sister site ConLang.SE, who specialise in this topic.
I see you have been asking a lot of questions that are related to tabletop RPG's, so I just wanted to mention that there is an RPG.SE in case you haven't come across that sister site. You might get some ideas for your current project by browsing their questions.
Welcome to WorldBuilding! This is a good question. As you are relatively new I just wanted to mention that there are lots of sites on the StackExchange network in case you didn't know about our sister sites. For example you might be interested in Writing.SE if you have some questions about how to write the story you are working on.
Nice question. While not about building a fictional world you might still be interested in checking out
[this question](link/to/a/relevant/question) on our sister site History.SE, which seems to be related to your current worldbuilding problem.
Some bad examples for when a question is on-topic on WorldBuilding and you want to point out that a dedicated Stack exists that makes it look like the question needs to be migrated:
This is a question for Astronomy.SE
You would get better answers on IPS.SE
GameDev.SE is better for questions like this.
While you might get answers here you should check out Islam.SE
Basically if you could change the link to the sister site to a link to Wikipedia or a blog article (and remove the sister site language) and you would think it's acceptable the comment is in line with the proposed comenting etiquette. Our sister sites are valuable resources, just like for example Wikipedia, and they should be treated as such in the cases where a question is on-topic on WorldBuilding.