The date of publication will help you decide whether a source is current or out-of-date.
Currency depends on your subject. Fast-changing fields within Computing and Science may require more up-to-date materials than Humanities and Social Sciences, which often have classic texts that it is important to read.
If your assignment has a historical context, then you will need to use both new and old information and, in this case, the older information is not out-of-date.
Check when a book was published by looking for the copyright date on the back of the title page, usually a few pages into the book.
Make sure you are reading the latest edition of a book by looking in the Library Search as textbooks are updated frequently.
Check whether the information you have found is up-to-date by searching carefully for any newer resources. If nothing has changed since something was written, then it will be useful no matter how old it is!
Journal articles almost always contain more up-to-date information than books.