MySQL ONLY. Confirmed on Rails 4.0.x, MySQL 5.6.x

group_date.rb

module ActiveRecord
  module Querying
    delegate :group_date, :to => :scoped
  end

  module QueryMethods
    def group_date(column_name = nil)
      return self if column_name.blank?

      # Rails uses a non standard time zone naming.
      # Let's get tz database standard which is used by many databases.
      # Using this instead of time offset or other approaches is necessary
      # in order to deal with daylight saving time automatically.
      zone = Time.zone.now.strftime('%z').insert(3, ':')

      # Rails stores timestamps without time zones by default.
      # So we must first set a zone to column_name (column_name at time zone 'UTC').
      # Note that even though Rails does that under the hood when you use ActiveRecord,
      # that's not applicable here.
      # Then, we set column to application's time zone so that the database can
      # convert timestamp to proper date.
      date = "date(CONVERT_TZ(#{table.name}.#{column_name}, '+00:00', \'#{zone}\'))"

      # Since converting date is not that straight-forward,
      # this will automatically return converted date.
      # We gotta convert output date to assure Rails won't convert it again.
      clone.group(date).select("#{date} as date_#{column_name}")
    end
  end
end