Hindu units of time are described in Hindu texts ranging from microseconds to trillions of years, including cycles of cosmic time that repeat general events in Hindu cosmology.^[1][2] Time (kāla) is described as eternal.^[3] Various fragments of time are described in the Vedas, Manusmriti, Bhagavata Purana, Vishnu Purana, Mahabharata, Surya Siddhanta etc.^[4][5][6]

Sidereal astrology maintains the alignment between signs and constellations via corrective systems of Hindu (Vedic)-origin known as ayanamsas (Sanskrit: 'ayana' "movement" + 'aṃśa' "component"), to allow for the observed precession of equinoxes, whereas tropical astrology ignores precession.^[7] This has caused the two systems, which were aligned around 2,000 years ago, to drift apart over the centuries.^[7][8]

Ayanamsa systems used in Hindu astrology (also known as Vedic astrology) include the Lahiriayanamsa and the Raman ayanamsa.^[9] The Fagan-Bradley ayanamsa is an example of an ayanamsa system used in Western sidereal astrology.^[9] As of 2020, zodiacal signs calculated using the Sri Yukteswar ayanamsa were around 23 degrees behind tropical zodiacal signs.^[7] Per these calculations, persons born between March 12 - April 12, for instance, would have the sun sign of Pisces.^[7] By contrast, persons born between March 21 - April 19 would have the sun sign of Aries per tropical calculations.^[10]

Sidereal Units:^{[11][failed verification]}

According to Sūrya Siddhānta:^[13][14]

Small units of time used in the Vedas:^[15][16]

The traditional lunar calendar system measures time based on the Moon's phases and its relation to the Sun. Unlike solar calendars, it uses units such as tithi (lunar day), pakṣa (lunar fortnight), māsa (lunar month), ṛitu (season), ayanam (half-year), and varsha (lunar year) to structure the year.^[17] This system was integral to ancient cultures for tracking time, planning festivals, and guiding agricultural practices. The following section provides an overview of these key time units and their relationships:^[18][19]

Tropical metrics are time units used to measure intervals based on the tropical year and related cycles. This system includes units such as ghaṭi (base unit), yāma (a period of 7.5 ghaṭis), and ahorātram (a full day comprising 8 yāmas).^[20] The following section explains these units and their approximate durations, offering insight into how time is organized in relation to the tropical year:^[21][a]

The table below contains calculations of cosmic Hindu units of time as experienced by different entities, namely humans, Pitris (forefathers), Devas (gods), Manu (progenitor of humanity), and Brahma (creator god). Calculations use a traditional 360-day year (twelve 30-day months) and a standard 24-hour day for all entities.