Kali yuga started 5000 years back is a well known fact. My question is did it start immediately on the day Lord Krishna ended His avatar or some time after that? I read some where that after Lord Krishna, a period of Yuga Sandhi existed for some time and only after that did Kali Yuga start. Can some one clarify based on scriptural evidence?
According to the mythological stories, the Kaliyuga did not start immediately after Krishna's death, but it started during the period of Parikshit's (Arjun's grandson) death.
From this wikipedia article on Pariskhit:
Once Parikshit went hunting in the forest. The demon Kali, the embodiment of Kali Yuga, appeared before him and asked permission to enter his kingdom, which the king denied. Upon insisting, Parikshit allowed him five places to reside: where there is gambling, alcohol consumption, prostitution, animal slaughter and gold. Kali smartly entered into Parikshit's golden crown and spoiled his thoughts. Parikshit entered the hut of a sage named Shamika as he was thirsty. He found the sage in deep meditation. He bowed to him several times but as there was no response he took a dead snake and threw it around the sage's neck. Later when the sage's son, Sringin, heard of this incident he cursed the king to die of snake bite on the 7th day.
However I cannot say that after how many years of Krishna's death, did Parikshit die.
Also one interesting fact which I have read from other sources: Duryodhana was embodiment of Kaliyuga. Due to his death, the Kaliyuga was at least delayed for 35 more years during which Yudhishtira was the king. This is analogical to to the modern time fact that, we have already crossed the time for the next ice age; but due to global warming, it's being delayed!
Krishna mentions in Mahabharat that the war time is not like krita, treta or Dwapara yuga but it is like kaliyuga. The below list the chapter from Udyoga parva.
Sanjaya said, 'Hearing these words of Karna, Kesava, that slayer of hostile heroes, spoke unto him these words smilingly, 'Do not the means of winning an empire recommend themselves to thee. O Karna? Wishest thou not to rule over the whole earth given by me to thee? The victory of the Pandavas, therefore, is very certain. There seems to be no doubt in this. The triumphal banner of Pandu's son, with the fierce ape on it, seems to be already set up. The divine artificer, Bhaumana, hath applied such celestial illusion (in its construction) that it standeth high, displayed like Indra's banner. Various celestial creatures of terrific shape, indicating victory, are seen on that standard. Extending for a yojana upwards and all around, that beautiful standard of Arjuna, resembling fire in radiance, is never, O Karna, when set up, obstructed by hills or trees. When thou wilt behold in battle Arjuna, on his car drawn by white steeds and driven by Krishna, applying Aindra, Agneya and Maruta weapons, and when thou wilt hear the twang of Gandiva piercing the welkin like the very thunder, then all signs of the Krita, the Treta, and the Dwapara ages will disappear (but, instead, Kali embodied will be present). When thou wilt behold in battle Kunti's son, invincible Yudhishthira, devoted to Yapa and Homa and resembling the very sun in brilliance, protecting his own mighty army and burning the army of his foes, then all signs of the Krita, the Treta, and the Dwapara ages will disappear. When thou wilt behold in battle the mighty Bhimasena dancing, after having quaffed the blood of Dussasana, like a fierce elephant with rent temples after having killed a mighty antagonist, then all signs of the Krita, the Treta, and the Dwapara ages will disappear. When thou wilt behold in battle Arjuna checking Drona and Santanu's son and Kripa and king Suyodhana, and Jayadratha of Sindhu's race, all rushing fiercely to the encounter, then all signs of the Krita, the Treta and the Dwapara ages will disappear. When thou wilt behold in battle the two mighty sons of Madri,--those heroic car-warriors, capable of breaking into pieces all hostile cars,--agitating, from the very moment when weapons will begin to clash, the army of Dhritarashtra's sons like a couple of infuriated elephants, then all signs of the Krita, the Treta and the Dwapara ages will disappear. Returning hence, O Karna, say unto Drona and Santanu's son and Kripa that the present month is a delightful one, and that food, drink, and fuel are abundant now. All plants and herbs are vigorous now, all trees full of fruits, and flies there are none. The roads are free from mire, and the waters are of agreeable taste. The weather is neither very hot nor very cold and is, therefore, highly pleasant. Seven days after, will be the day of the new moon. Let the battle commence then, for that day, it hath been said, is presided over by Indra. Say also unto all the kings that have come for battling that I will fully accomplish the desire cherished by them. Indeed, all the kings and princes that are obedient to the orders of Duryodhana, obtaining death by weapons, will attain to an excellent state.'"