First, Sri Krishna did not rule Magadha, Jarasandha did. Even during the late Dwapara Yuga, Magadha was one of the most powerful empires of Ancient India.
Once the Saraswathi River dried on the surface (5000 to 3000 BCE) and became an underground river (antarvahini), many brahmans moved to Naimisaranya in Uttar Pradesh. These were headed by Saunaka and the others. They received history as recorded in the Mahabharata and the Puranas from Veda Vyasa’s students. (Romaharsha and Ugrasrava Sauti and others). This was the beginning of the Kaliyuga. (See How many kinds of Yugas are there?)
Designated chapters of each of the puranas, and a seperate bhavishya purana, were set aside for “bhavishya” ie for those incidents that took place after the Mahabharata war. They were updated in future tense to keep with the voice of the narrator. But they took more interest in the rulers and history of Magadha.
The Yadavas moved south along the west coast (Maharashtra, Karnataka). The Ikshvakus went back eventually to the east coast (Andhra Pradesh and Tamil Nadu). I think the Vasus were connected with Bengal.
A smaller set of people also moved to and settled in the Sindhu Valley. ( See : Archaeological Connections)
The Purus continued in Hastinapur and at least initially in Gandhar (Takshashila).
The calendar we must use to understand the time of the Manus, prior to Vaivaswatha Manu is the Manvantaras, the region is Dravida. One major event was the flood of the Matsya Avataram, (See Manus and Manvantaras)
The calendar that we must use to understand the period of the Ikshvakus (Sri Rama) and Yadavas (Sri Krishna) from the Vedic period, Ramayana War and till the Bharata War is the Yuga Calendar. The region is all over India. See (Date of Sri Rama, Date of the Mahabharata War, Date of Veda Mantras)
The calendar that we must use to understand the period from the Bharata war to the Muslim Rule are the different Sakas as well as the Yugas and the Saptarishi Calendar. The region is in and around Magadha. (See : How many kinds of Sakas (Eras) are there? How many kinds of Yugas are there? Saptarishi Calendar)
The document below is a collection of my various articles on this subject and will be updated as time flows past space in our perception. (Space Time Continuum, Time Line or Time Circle?, Two kinds of time)
For other areas, other texts and possibly other calendars must be used.
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