JsonConvert.SerializeObject(pd, new IsoDateTimeConverter() { DateTimeFormat = "yyyy-MM-ddTHH:mm:ss" })

Convert DateTime to ISO 8601 String in C#

https://stackoverflow.com/questions/18635599/specifying-a-custom-datetime-format-when-serializing-with-json-net

https://stackoverflow.com/questions/42811410/how-to-get-json-net-to-serialize-date-time-to-iso-8601