I would like to ask how to disable the editing of time and date published? Also, I would like to ask if it's possible for people who doesn't own the blog to post, not just comment, but post their own posts in the blog. Please and thank you.
There is no straightforward way to do this-- at least not one that I see. That is, there don't seem to be any hooks specifically intended for this task. You still have options though. This is what I'd do:
First, deny changes.
You could add conditions for administrators if you wanted, or conditions for future posts, but the code as-is will completely remove any ability to edit the publication dates.
You will still see the form pieces though, so hide those with css.
These functions can go in your theme's