In the beginning there was only Divine Energy. The Creator, the Preserver and the Destroyer. This is the first step to all creation. A thought here. I feel that a year for us is defined by our Solar system with Sun being the center of the system.
Lets crunch some numbers now.
One Ahoratr constitutes a day. Three hundred and sixty human years equal one God year. Thus 12,000 God years equal 4,320,000 man years. Satya Yug, Tret Yug, Dvapar Yug and Kali Yug. In addition ,there are other time periods between these four Yugas.