section_path: Definitions/In the International System of Units
section_name: In the International System of Units
paragraph_text: In the International System of Units
  
  In the International System of Units (SI), a day not an official unit, but is accepted for use with SI. A day, with symbol d, is defined using SI units as 86,400 seconds; the second is the base unit of time in SI units. In 1967–68, during the 13th CGPM (Resolution 1), the International Bureau of Weights and Measures (BIPM) redefined a second as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium -133 atom ". This makes the SI-based day last exactly 794,243,384,928,000 of those periods.