The expiration of a copyright is more complex than that of a patent. Historically the United States has specified terms of a number of years following creation or publication; this number has been increased several times. Most other countries specify terms of a number of years following the death of the last surviving creator; this number varies from one country to another (50 years and 70 years are the most common), and has also been increased in many of them. See List of countries' copyright length. Legal traditions differ on whether a work in the public domain can have its copyright restored. Term extensions by the U.S. and Australia generally have not removed works from the public domain, but merely delayed the addition of works to it. By contrast, a European Union directive harmonizing the term of copyright protection was applied retroactively, restoring and extending the terms of copyright on material previously in the public domain.