lib/mandrill_dm/message.rb
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
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
mandrill expects `send_at` in UTC as `YYYY-MM-DD HH:MM:SS`