Attempted fix for overzealous refactoring.

a37c4e1d99ce7fda39bf292584730ae6af640915 factored out

    read_attribute(:datetime_value)

to #datetime_value.  Unfortunately, this shadows ActiveRecord's
definition of #datetime_value (which is auto-generated).  Hypothesis:
this is causing things like timezone conversion to be eliminated from
the attribute reader.