A patent may expire 17 years after its application if said application was filed by the 7th of June 1995 and the patent was issued before the 8th of June 1978. However, if the patent application was filed by the 7th of June 1995 and the patent was issued after the 8th of June 1978, then the patent expires either after 17 years from the date of issuance or 20 years from the earliest filing of the application, whichever is farther into the future. If a patent application is filed after the 7th of June 1995, then it expires exactly 20 years after its filing date. These laws apply only the US under the General Agreement on Tariffs and Trade. Countries may have different patent laws that consist of varying patent expiration dates.
In an effort to bridge the gap between US businesses and international businesses, the Patent and Trademark Office changed the patent expiration dates in order to match the typical 20-year term given by other countries. This was agreed upon by the US and other countries during the Uruguay round of the General Agreement on Tariffs and Trade on the 8th of December 1994.
The varying patent expiration dates aforementioned are part of the mechanisms that Congress provided in order to transition into the new patent expiration date. Patent laws also stipulated a maintenance fee for a utility patent as part of the conditions of patent expiration. Conditions in patent expiration also include a maximum of 5 years due to marketing delay or FDA regulation.