if >= && date range between [on hold]

My code pings a list of urls, it then stores the results in a database. It then sends a generic email when a ping within the database is below 1000ms. I want the code to be able to send an email only if the ping was within the last 24 hours. What can I use for the time frame?

DateTime now = DateTime.Now;
DateTime yesterday = now.AddDays(-1);


                    if ((int)response >= 1000)
                    {
                        SendEmailPageloadFailed();
                    }
                }