code: def send_at_formatted_string(obj)
        return obj if obj.is_a?(String)
  
        obj = obj.to_time if obj.is_a?(DateTime)
        return obj.utc.strftime('%Y-%m-%d %H:%M:%S') if obj.is_a?(Time)
  
        raise ArgumentError, 'send_at should be Time/DateTime or String'
      end
docs: mandrill expects `send_at` in UTC as `YYYY-MM-DD HH:MM:SS`