To give credit where credit is due.
So that readers can go back to the original sources of facts and opinions for themselves.
To avoid violating copyright.
APA (American Psycological Association)
MLA (Modern Languages Association)
ICS (International Council of Sciences)
Use the current date.
Leave the date blank.
Make your best guess.
Not use the source, because a valid source always lists its publication date.
A number or statistic.
A fact that is common knowledge.
Use the Misc. Source format.
Submit the citation information for that source separately.
Use citation format for the kind of source that is most similar to it.
Cite it as if it were a book.