This problem exists because Marshal.load calls Date.allocate, which
uses a SimpleDateData.  There doesn't seem to be any support for
taking an existing Date instance and converting it from SimpleDateData
to ComplexDateData.  Work around this issue by making Date.allocate
use a ComplexDateData.  This causes problems in Date#initialize,
so remove the Date#initialize method (keeping the date_initialize
function, used internally for Date.civil). Alias Date.new to
Date.civil, since they do the same thing.

https://github.com/ruby/date/commit/6bb8d8fa0f