Vishnupurana 2.8.71

2.8.71

saṃvatsarādayaḥ pañca caturmāsavikalpitāḥ
niścayaḥ sarvakālasya yugam ity abhidhīyate //

Fifteen days of thirty Muhúrttas each are called a Paksha (a lunar fortnight); two of these make a month; and two months, a solar season; three seasons a northern or southern declination (Ayana); and those two compose a year. Years, made up of four kinds of months, are distinguished into five kinds; and an aggregate of all the varieties of time is termed a Yoga, or cycle. The years are severally called Samvatsara, Parivatsara, Idvatsara, Anuvatsara, and Vatsara. This is the time called a Yuga.