(1) Subject to the provisions of this Constitution, a person shall hold the office of President until -
(a) when his successor in office takes the oath of that office;
(b) he dies whilst holding such office; or
(c) the date when his resignation from office takes effect;
(d) he otherwise ceases to hold office in accordance with the provisions of this Constitution.
(2) Subject to the provisions of subsection (1) of this section, the President shall vacate his office at the expiration of a period of four years commencing from the date, when -
(a) in the case of a person first elected as President under this Constitution, he took the Oath of Allegiance and the oath of office; and
(b) in any other case, the person last elected to that office under this Constitution took the Oath of Allegiance and oath of office or would, but for his death, have taken such Oaths.
(3) If the Federation is at war in which the territory of Nigeria is physically involved and the President considers that it is not practicable to hold elections, the National Assembly may by resolution extend the period of four years mentioned in subsection (2) of this section from time to time; but no such extension shall exceed a period of six months at any one time.
Section 135 of the Constitution of the Federal Republic of Nigeria