The concept of Hindu time keeping is mind-blowing and even beyond the purview of modern science. However Hindus have deep faith in Vedic concept of time. The biggest unit of Hindu time is Maha Manvantara which is equals to 311.04 trillion years. According to Vedic texts, the creation and destruction of the universe is a cyclic process which repeats itself forever. Each cycle starts with the birth and expansion (lifetime) of the universe which equals to 311.04 trillion years followed by its complete annihilation which also prevails for the same duration of 311.04 trillion years.
The Hindu time keeping is cyclic and any bigger cycle contains smaller cycle inside it. The most familiar cycle of time is Mahayuga which contains four Yuga(s).
One cycle of the above four Yugas is one Mahayuga (4.32 million solar years = 10x)
1000 Mahayuga(s) = A day of Brahma (excluding night)
= 4.32 billion solar years
= 1000 * 10x
= X = 1 Kalpa
1 full day of Brahma = 2 Kalpa = 2X
1 year of Brahma = 360 * 2X = 3.1104 trillion solar years
100 years of Brahma = 311.04 trillion solar years
= Life Span of Brahma
= 1 Maha Kalpa
= 1 Para
= 2 Para Ardha
= 1 Maha Manvantara
X = 1 Kalpa = 14 Manvantara = 1000 Mahayuga
Currently, 50 years of Brahma have elapsed and we are in the first Day of the 51st year. This Brahma's day, Kalpa, is named as Shveta-Varaha Kalpa. Within this Day, six Manvantara(s) have already elapsed and we are in the seventh Manvantara, named as - Vaivasvatha Manvantara or Sraddhadeva Manvantara.
Within the Vaivasvatha Manvantara, 27 Mahayuga(s) and the Krita, Treta and Dwapara Yugas of the 28th Mahayuga have elapsed. We are in the Kaliyuga of the 28th Mahayuga. This Kaliyuga began in the year 3102 BC in the proleptic Julian Calendar. Since 50 years of Brahma have already elapsed, we are in the second Parardha, also called as Dvithiya Parardha.
Tithis when 4 Yuga repeats are known as Yugadi, when 14 Manvantara repeats are known as Manvadi and when 7 Kalpa repeats are known as Kalpadi. Hence Hindu calendar marks 4 Yugadi Tithis, 14 Manvadi Tithis and 7 Kalpadi Tithis for religious rituals.
Yuganta (i.e. the day when Yuga gets over) are also marked by Hindu Calendar.