MPEP 2700 deals with patent terms and extensions.
Patent terms are dependent on the type of patent and the date of filing. For a utility (non-provisional) patent filed before June 8th, 1995 the patent term is the greater of 20 years from the filing date or 17 years from the issue date. For utility (non-provisional) applications on or after June 8th, 1995 the patent term begins on the issue and and lasts for 20 years from the filing date. Any claims of priority to foreign or provisional patents are not included when determining the filing date of the patent term purpose. For patents resulting from international applications, their term ends 20 years from the date of filing the international application.