The Hindu Lunisolar calendar includes a sixty year cycle wherein each year is called Samvatsara and given a unique name (Apr 14, 2012 started the Nandana Samvatsara). This 60 Year cycle itself is called the Guror Mana (aka Bruhaspatya Mana) i.e. it is named after Jupiter. I have pointed out elsewhere that Jupiter orbits the Sun and hence the Earth once every 12 years. Why then multiply this by 5 to arrive at a sixty year cycle?
One speculation is that this cycle is based on the Great Conjunction, i.e. the conjunction of Jupiter and Saturn, two of the biggest planets in the solar system. Such a conjunction occurs once every 20 years approximately but in different segments of the Zodiac; the Conjucntion returns to the same segment of the Zodiac (or very near it) once every 60 years.
Confused? Take a look at this animation of the orbital motion of Jupiter and Saturn around the Sun.
Play around with the controls and see how the Great Conjunction keeps moving around the Zodiac and returns near its starting point (wherever you choose to start from) every sixty years approximately.
Now, there is no explicit statement in the Surya Siddhanta or any of the Hindu astronomical works that the Guror Mana is indeed based on the Great Conjunction, hence this idea is entirely speculative in nature. Yet, given the other alternative of believing a completely arbitrary multiplication factor of 5 was applied to the orbital period of Jupiter, I will take this Great Conjunction hypothesis any day.