There are two methods that are usually used to calculate kāla horās:
- 24 equal horās from sunrise to sunrise;
- 12 horās from sunrise to sunset and 12 horās from sunset to sunrise.
Option (1) is described in the following works:
- “Hindu Electional Astrology”, section 3.3 by V. K. Shridhar (2002)
- “Brihat Parasara Hora Sastra”, volume 1, verse 27.13 by R. Santhanam (1984)
- “Vedic Astrology: An Integrated Approach”, section 1.3.11 by P. V. R. Narasimha Rao
Option (2) is described in the following sources:
- “Muhurta Traditional and Modern”, chapter 2 by K. K. Joshi (2003)
- “Light on Life: An Introduction to the Astrology of India”, chapter 7 by H. de Fouw and R. Svoboda (2003)
It should be noted, however, that in “Light on Life” the authors conclude the description of kāla horā calculation by saying that a good astrologer should use his or her intuition to decide whether option (1) or (2) should be used on each given occasion, so this source is not univocal in this regard.
Chakra Darshana implements option (1), because there is currently more evidence supporting it, although further research is definitely required.