I have found a book The age of Shankara by T. S. NArAyaNa wherein in the introduction the author writes how Christian orientalists deliberately have been shifting birth dates of the great Indian personalities like Adi Sankara, Gautam Buddha etc to suit their Christian chronology. In one passage he writes
But if take for granted that the MahABArata war took place 37 years before the commencement of KalIyuga, which appear to me the most correct view and which is admittedly the opinion of all the Hindu writers from the ancient times, Gautama Buddha must be places before 2000 B.C. This view is fully supported by all the authentic PurANas and ItIhAsas, which are beginning to be recognised as contributing genuine historical information about Ancient India and according to which the Buddha is to be placed not later than 1800 B.C.
I wanted the proper reference from PurANas and ItihAsas which checkmark the above claims of the author. Where are they?