Prof. Anand Sharan who proposed 2156 BCE for the year of Mahabharata war writes…
“Shukla Paksha 8 can be ruled out as Bhishma Ekadasi occurs in the Shukla Paksha and Bhishma obviously obtained Nirvana after that in the Krisna Paksha.”
He was writing in the context of my blog article, here.
I have written previously in the context of ‘traditional tithis’ for celebration of ancient events and their value (and also limitations/conflict) with internal evidence of the epics.
If one believes the traditional tithi of Margashirsha Shukla 11 as the first day of Mahabharata war and hence Gita-Jayanti, that would mean Bhishma fell in the battle around Margashirsha Vadya 5, which in turn means these believers would have to discard the second traditional tithi of Bhishma-Nirvana as that of Magha Shukla 8. This is because the duration of Bhishma on the bed of arrows would only add up to 48 days.
If one believes the traditional tithi of Magha Shukla 11 as the day of Bhishma Ekadashi, one would not have to give up the belief of Gita-Jayanti (= Margashirsha Shukla 11) but would have to give up the belief of Bhishma Nirvana (= Magha Shukla 8).
Per popular but falsified claim of Bhishma on bed of arrow for 58 nights, the tithi, in this case, would be Magha Vadya/Krishna 3 or 4.