The Vedic Dindarshika (calendar) provides a mean to get introduced with the ancient tradition of knowing days or Tithis based on lunar phases, along with traditional names of the fortnights, months and seasons. The simple circular mandala like design elegantly represents the periodic nature of the eternal Time. Precisely crafted moon phases infuse the curiosity of young minds to compare them with the real moon every day, and with little experience one can predict the Tithis, Paksha with a glance of the real moon. That is all, a learning for the life-time accompanied with fun!
The detailed description about the Time units is not to prove any mathematical feat or boggle the young minds, rather it is presented with the motive of letting the young generation know about the insignificance of our (human’s) existence in this myriad universe that gets created and destroyed every single day of Brahma. We hope it will convey to them how to utilize the short amount of time that we got in this valuable life.
The concept of Time has always been cyclic in śāstra (Indian scriptures), not limited to only cycle of seasons of a year but the creation and destruction of the universe and millions and millions of it. It will be unfathomable for some to even imagine the extent to which cyclic nature of Time is described in śāstra.
When modern science dares only to speculate the relativity of time, śāstra defined it precisely way back in time. There are different time measurement units for different beings, such as Pitṛs (forefathers), Deva (demigods) and Lord Brahma.
There are smaller units to measure the time of very minute scale such as Truṭi, Nimisha and there are massive units to measure a day and life span of the creator of the universe, Lord Brahma (i.e. Mahā-Kalpa spans in trillions of our solar years).
So to start with more familiar units of Days and Nights. Fifteen (15) pairs of day-night comprises a Pakṣa, two such Pakṣa (Śukla Pakṣa & Kṛiṣhṇa Pakṣa) together forms a lunar month, i.e. Māsa. Two (2) Māsa makes a Ṛitu (or season), so in a year there are 6 seasons, unfortunately most next generation is introduced to only 3-4 seasons. Three (3) such Ṛitu together are called as an Ayana, and so we have Uttarāyaṇa and Dakshināyaṇa in a Samvatsara (year).
Calculating the numbers of time units for Deva and Brahma in terms of our solar years will really cost big chunk of our time! In short, there is a cycle of four Yugas comprising a Mahā-Yuga. Again the cycle of such 1000 Mahā-Yuga makes up a day of Brahma (Kalpa). Lord Bramha’s life span is considered to be of one hundred Brahma years or one Mahā-Kalpa, that equates something like 313,528,320,000,000 human years. There are many more details involved to it like Sandhya or Saṃdhi Kāla after every Yuga and other units like Manvatara, etc. but that is left for the seekers to explore.
In Bhagavad Gita also, Lord Krishna said,
सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदुः ।
रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः ॥ ८.१७ ॥
sahasra-yuga-paryantam ahar yad brahmaṇo viduḥ
rātriṁ yuga-sahasrāntāṁ te ’ho-rātra-vido janāḥ
“By human calculation, a thousand Yugas taken together form the duration of Brahmā’s one day. And such also is the duration of his night.”