Introduction to Cosmology - Experimental Elementary Particle
... defined as the time it takes the Earth to go once around the Sun. One
year is approximately equal to 3.2 × 107 s. In cosmological context, a year is
frequently an inconveniently short period of time, so cosmologists frequently
use gigayears (Gyr), equal to 109 yr, or 3.2 × 1016 s. For example, the a ...