Yugas and Eras
From the smaller unit of day, we went to the larger units, namely months and years. Now we'll deal with something which is VERY LARGE. This is called Yuga. There is an interesting reason why the yugas were thought of for a larger unit of time. In the past, our ancestors didn't deal with real numbers (numbers having decimal points like 365.25) but instead they dealt with fractions. So there was a need for expressing the various quantities like the sidereal period of the Sun (365.2563 days), sidereal period of the moon (27.32166 days), lunar month (29.530589 days) in fractions. So they devised the Yuga system. A yuga is a very large unit of time and the number of lower units like the sidereral period of the Sun, sidereal period of the Moon, Lunar month, time for revolutions of various planets, etc. in a Yuga are integers. In other words, a Yuga is a common multiple of all these fractional quantities. There are still larger units of time like the Maha Yugas. I bet you'll enjoy browsing through more about Mahayugas.
There are many eras, which are widely used in India. The beginning of an era is chosen to commemorate an important event like the coronation of a king, victory over some enemy and so on. Most of the eras are used in both lunar and solar calendars. The most important of them is the Saka era (called Salivahana Saka, Saka Samvat etc., in different parts of India). The epoch of an era is nothing but the starting point of an era. The epoch of the lunar Saka era is Chaitra Sukla Prathama of the Kaliyuga year (lunar) 3179 (elapsed) which happened to be March 3rd, 78 AD. The epoch of solar Saka era is the Mesha sankranti in the same year, 78 AD. The Saka era is widely used throughout India for all astronomical calculations and Panchanga making.
What's the use of just knowing the units of time? If you are asked the question - How many days have elapsed since such and such date? what would you do? Here's the answer in the form of ahargana.