Rails business hours query for active record

I have two models

OfficeTimeing < ActiveRecord::Base
  belongs_to :office
end


Office < ActiveRecord::Base
  has_many :office_timings
end

with two fields opening_time and closing_time these fields have string values like "09:00 AM" I want a query how can I find currently open offices I want something like this.

Office.joins(:office_timings).where("Datetime.now > office_timings.opening_time AND office_timings.closing_time > DateTime.now")