You wouldn't be able to close all date and time formatting questions via one answer, because they won't all be needing the same thing.
For example, two questions which sound similar but have radically different "good" answers:
- How should I format my DateTime to display it to a user?
- How should I format my DateTime to include it in a database query?
In the second case, the best answer is "don't include the DateTime in your SQL to start with; use a parameterized query" - but that's completely irrelevant to the first question (which would need to find out about the user's culture etc).
I think it's reasonable to pick a well-asked question, write a comprehensive answer to just that question and then close very similar questions as duplicates. I don't think this belongs in the tag wiki, but a link to that question could belong in the tag wiki.
(I've been considering writing "hopefully definitive" questions for very common concerns in the same way.)