Newtonsoft.Json.Converters.IsoDateTimeConverter timeCover = new Newtonsoft.Json.Converters.IsoDateTimeConverter();
timeCover.DateTimeFormat = "yyyy-MM-dd HH:mm:ss";

string jsonData = JsonConvert.SerializeObject(XXXXModel, timeCover);

//簡寫
string jsonData = JsonConvert.SerializeObject(XXXXModel, Formatting.Indented, new Newtonsoft.Json.Converters.IsoDateTimeConverter() { DateTimeFormat = "yyyy-MM-dd HH:mm:ss.fff" })