Bhishma passed away on this day (31 January 5560 BCE) and thus 7576 years ago.
Day of Winter Solstice coincided with the day of 30/31 January during 5560 BCE (calculations are per Julian calendar computations, extended in antiquity).
What is the significance of Bhishma Nirvana?
We have following pieces of information from the Mahabharata text
(1) Bhishma passed away on the day of (or day after) winter solstice
(2) Bhishma was lying on the bed of arrows (from the 10th day of Mahabharata war and until the day of winter solstice) and this duration was greater than 92 days.
(3) The Mahabharata war began sometime after Kartika Pornima
Calculations: Back of the envelope version
Back of the envelope calculations allow us to estimate the timing of this incident. (Keep in mind that the date of Mahabharata war 16 October – 2 November 5561 BCE, has been arrived at (by Dr. P V Vartak) and validated by yours truly, via independent observations of Mahabharata text. (independent of Bhishma Nirvana observations))
In our times (2016) the point of Median Tithi for the day of winter solstice is Margashirsha Vadya/Krishna Saptami.
Considering the most conservative estimate of day for the first day of Mahabharata war to be that of Margashirsha Shukla 1 (or day before on Amawasya), ….
and thus Margashirsha Shukla 9 for the 10th day of war, when Bhishma fell down in the battlefield…..
The point of Median Tithi that coincided with the day of winter solstice, during Mahabharata time would be (~20 Tithis of Margashirsha + 29.5 Tithis of Pausha + 29.5 Tithis of Magha + AT LEAST 13 Tithis of Phalguna = ~ 92 days)
This leads to estimate of Median Tithi (for the day of winter solstice) to be that of Phalguna Shukla 13.
This amounts to a MINIMUM gap of (Phalguna Shukla 13 – Margashirsha Vadya/Krishna 7 ) =~ 81 days.
It takes about 72 years for a Median Tithi to shift by a day and this gives us,
A lower limit for occurrence of Bhishma Nirvana to be ~ 72 x 81 = 5832 ~ 6000 years ago!
at least before ~4000 BCE!
Thus, Bhishma-Nirvana happened sometime before 4000 BCE.
Of course, based on independent evidence (independent of Bhishma Nirvana references of Mahabharata text) we already know that it did occure on 30/31 January 5560 BCE.
The key point is that multiple AND independent sets of references of Mahabharata text:
(1) AV observation
(2) Flooding of Dwarka combined with geological evidence
(3) astronomy evidence of Bhagavad Gita
(4) 23+ references of Bhishma Nirvana),
all lead to the conclusion that the Mahabharata war occurred in 6th millennium BCE!