// Format a datetime object to include 'T' seperator and offset
DateTimeOffset now = DateTimeOffset.Now;
paymentDate = new DateTimeOffset(now.Year,
                                 now.Month,
                                 now.Day,
                                 0,
                                 0,
                                 0,
                                 new TimeSpan(1, 0, 0))
                                 .ToString("yyyy-mm-dd'T'HH:mm:ssK");

// Example output 2019-09-04T00:00:00+1:00