Added timestamp string converter

CryptoExchange.Net/Converters/TimestampStringConverter.cs

@@ -0,0 +1,44 @@
+using System;
+using Newtonsoft.Json;
+
+namespace CryptoExchange.Net.Converters
+{
+    /// <summary>
+    /// converter for datetime string (yyyymmdd) to datetime
+    /// </summary>
+    public class TimestampStringConverter : JsonConverter
+    {
+        /// <inheritdoc />
+        public override bool CanConvert(Type objectType)
+        {
+            return objectType == typeof(DateTime);
+        }
+
+        /// <inheritdoc />
+        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
+        {
+            if (reader.Value == null)
+                return null;
+
+            var value = reader.Value.ToString();
+            if (value.Length == 8)
+                return new DateTime(int.Parse(value.Substring(0, 4)), int.Parse(value.Substring(4, 2)), int.Parse(value.Substring(6, 2)), 0, 0, 0, DateTimeKind.Utc);
+            else if(value.Length == 6)
+                return new DateTime(int.Parse(value.Substring(0, 2)), int.Parse(value.Substring(2, 2)), int.Parse(value.Substring(4, 2)), 0, 0, 0, DateTimeKind.Utc);
+
+            throw new Exception("Unknown datetime value: " + value);
+        }
+
+        /// <inheritdoc />
+        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
+        {
+            if (value == null)
+                writer.WriteValue((DateTime?)null);
+            else
+            {
+                var dateTimeValue = (DateTime)value;
+                writer.WriteValue(int.Parse($"{dateTimeValue.Year}{dateTimeValue.Month}{dateTimeValue.Day}"));
+            }
+        }
+    }
+}