Enhanced the UnixDateTimeConverter to...

src/Stripe.net/Infrastructure/JsonConverters/UnixDateTimeConverter.cs

@@ -80,7 +80,17 @@ public override object ReadJson(JsonReader reader, Type objectType, object exist
                 throw new JsonSerializationException(string.Format("Unexpected token parsing date. Expected Integer or String, got {0}.", reader.TokenType));
             }
 
-            if (seconds >= 0)
+            // The maximum allowed value of seconds allowed for DateTime.AddSeconds(double d). If this value is converted (seconds since Unix Epoch Time), it is equivalent to December 31, 9999 23:59:59 UTC.
+            const long maxSecondsForDateTime = 253402300799;
+
+            // Determine if this value is milliseconds after Unix Epoch
+            if (seconds > maxSecondsForDateTime)
+            {
+                // Modify this value to convert it from milliseconds (since the Unix Epoch) to be seconds, by truncating the milliseconds.
+                seconds /= 1000;
+                return DateTimeUtils.UnixEpoch.AddSeconds(seconds);
+            }
+            else if (seconds >= 0)
             {
                 return DateTimeUtils.UnixEpoch.AddSeconds(seconds);
             }

src/StripeTests/Infrastructure/JsonConverters/UnixDateTimeConverterTest.cs

@@ -0,0 +1,34 @@
+namespace StripeTests.Infrastructure.JsonConverters
+{
+    using System;
+    using Newtonsoft.Json;
+    using Stripe;
+    using Stripe.Infrastructure;
+    using Xunit;
+
+    public class UnixDateTimeConverterTest
+    {
+        [Fact]
+        public void Deserialize_UnixEpochSeconds()
+        {
+            var json = @"{ ""created-date"": 1625097600 }"; // Represents July 1, 2021
+            var obj = JsonConvert.DeserializeObject<TestObject>(json);
+            Assert.Equal(new DateTime(2021, 7, 1, 0, 0, 0, DateTimeKind.Utc), obj.CreatedDate);
+        }
+
+        [Fact]
+        public void Deserialize_UnixEpochMilliSeconds()
+        {
+            var json = @"{ ""created-date"": 1625097600000 }"; // Represents July 1, 2021
+            var obj = JsonConvert.DeserializeObject<TestObject>(json);
+            Assert.Equal(new DateTime(2021, 7, 1, 0, 0, 0, DateTimeKind.Utc), obj.CreatedDate);
+        }
+
+        private class TestObject : StripeEntity<TestObject>
+        {
+            [JsonProperty("created-date")]
+            [JsonConverter(typeof(UnixDateTimeConverter))]
+            internal DateTime CreatedDate { get; set; }
+        }
+    }
+}