Siddhanta Shiromani (Crown of Treatises)

When it is said that the Civil Day sāvana is a "Terrestrial Day" original: bhūdina, it should be understood that the classification of months and years here follows the previously established definitions. Now, the term "Civil Day" is used as a general indicator original: upalakṣaṇam. This means that the interval between any two risings of whichever planet is being considered is the "Civil Day" for that specific planet. As for what has been stated regarding the Civil Days, the following explains their nature:

The Sidereal Day nākṣatra refers to the "rotation of the circle of stars" original: bha-bhramaṇa. The rotation of the stars, beginning with Aśvinī the first asterism of the Hindu zodiac and so on—the time it takes for this rotation to occur is a Sidereal Day. This means that from the moment a particular star rises until that same star rises again, that duration is a Sidereal Day. This Sidereal Day consists of sixty units original: ṣaṣṭī; referring to ghaṭikās, where 60 ghaṭikās equal 24 hours. Here too, the calculations for months and years are to be understood by the previous definitions.

Accordingly, in the Sūrya Siddhānta a primary authority on ancient Indian astronomy, because of the mention of "sixty," the Sidereal Day-and-Night is proclaimed as such; thirty of these make a month. Furthermore, it is said that the Sidereal Day is defined by the constant rotations of the zodiac original: bhacakra-bhramaṇa.

Now, the author describes the measurement of the Ages yuga, such as the Kṛta and others, through the measurement of days:

These ages are adjusted by their own specific parts original: aṃśakaiḥ mixed with their own solar proportions. When these are combined, they form the full Yuga. The movements of the ages like the Kṛta and others are calculated by these Solar Years sūrya-vatsara. It is taught that one complete transit of the Sun through the zodiac is a Solar Year. By such defined Solar Years—numbering 4,320,000—[the Great Age is measured].