Bhagavatapurana 5.22.5

5.22.5

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atha sa eṣa ātmā lokānāṃ dyāv-āpṛthivyor antareṇa nabho-valayasya kālacakra-gato dvādaśa māsān bhuṅkte rāśi-saṃjñān saṃvatsarāvayavān māsaḥ pakṣa-dvayaṃ divā naktaṃ ceti sapādarkṣa-dvayam upadiśanti yāvatā ṣaṣṭham aṃśaṃ bhuñjīta sa vai ṛtur ity upadiśyate saṃvatsarāvayavaḥ //

5. Further that Lord of worlds standing in relation to Kalachakra in the middle of the sky midway between the Earth and heaven holds his sway on the twelve months called the twelve signs, the divisions of a year; a month consists of two fortnights which are the day and night (of Pitris) and it is also said to be two stars and a quarter (*i. e.*, the period called a month is the time taken by the sun to traverse two constellations and a quarter in space); the time by which he traverses one-sixth of the orbit is said to be a season or *Rithu* forming a distinct part of the year.