Atlantis existed well before the ancient Sumerians, Babylonians, Egyptians and others had settled on a 365 day calendar.
The Atlanteans worked on a 360 day calendar with 12 months, each containing 3 ten day weeks. It has been suggested that a pole shift occurred (perhaps at the same time as Atlantis disappeared?) which lengthened the year by five days.
Whatever the cause, gradually the great civilsations of antiquity added the five days.
Atlantean terms used in "Through All Eternity":-
zytur = year triton = month tonn = week
So much knowledge literally at our fingertips these days makes us very fortunate. Google 'epagomenal days' for some fascintaing reading.