Lets try to decode logic behind this time calculation. We all know that one complete day is of 24 hours which consists of a day and a night. As most of the human population lives near equator we observe length of day and night to be approximately 12 hours. But if this is this phenomenon obseved across the globe?

Let's see. Let's consider the case of the North Pole. At the North Pole Sun rises once in a year and Sun can be seen above horizon for 6 months at a stretch. Similarly, night can be observed for 6 months at a stretch at North Pole. So, a complete day at the North Pole of the Earth is around 360 civil days approximately (if we consider 30 days month then civil year is approximately 360 civil days).

- As per Veda's and Purana's, one human year is considered as one complete day of Devta's. In one human year Earth's axis completes one circle. In first half of this circle, Northern hemisphere of Earth moves close to Sun which is called as "Uttarayan" in Veda's and Purana's. In second half of this circle, Southern hemisphere of earth moves close to Sun which is called as "Daskshinayan" in Veda's and Purana's.

- During Uttarayan days start getting longer and night's in Northern Hemisphere. During Dakshinayan days start getting shorter than night's in Northern Hemisphere. One human year consists of one Uttarayan plus one Dakshinayan. 

- In Veda's and Purana's, Uttarayan is considered as day time of Devtas and Dakshinayan is considered as night time of Devtas. 

- Devta's are natural (cosmic) powers like Pruthvi (Earth), Agni (Fire), Jal (Water), Vayu (Air), Aakash (Space). As life of humans is very much dependent on these natural powers, change in natural cycle of these natural powers is celebrated and welcomed.

When observed from Earth's surface, Sun takes approximately 360 days to complete one revolution around our stellar sphere. Similarly, planet Jupiter takes approximately 12 years to complete one revolution around our stellar sphere. The fact is planet Jupiter takes approximately 12 years to complete 1 rotation around the Sun.