Dear Ramanaduli,
Yes, Jnanis like Bhishma decided their date of leaving the body,
for the commencement of Uttararayna. But dying on Ekadasi
day and getting cremated on Dwadasi day, could be an extension
of these ideas, perpetrated by Vaishnavites. Chanting God's
name, while dying, is the idea of having a noble thought at the
time of death, so that the mind and pranana would go with that
thought for a better state. Chanting God's name will not come to
one suddenly, at the time of death, unless he is chanting the names
at all times in his life.
Arunachala Siva.