diff --git a/modules/swagger-codegen/src/main/resources/CsharpDotNet2/ApiClient.mustache b/modules/swagger-codegen/src/main/resources/CsharpDotNet2/ApiClient.mustache
index 1d3fa50a284..6d5e45c186e 100644
--- a/modules/swagger-codegen/src/main/resources/CsharpDotNet2/ApiClient.mustache
+++ b/modules/swagger-codegen/src/main/resources/CsharpDotNet2/ApiClient.mustache
@@ -143,6 +143,9 @@ namespace {{packageName}}.Client
         public string ParameterToString(object obj)
         {
             if (obj is DateTime)
+                // Return an ISO 8601 formatted string, also known as a Round-trip date/time pattern
+                // https://msdn.microsoft.com/en-us/library/az4se3k1(v=vs.110).aspx#Anchor_8
+                // For example: 2009-06-15T13:45:30.0000000
                 return ((DateTime)obj).ToString ("o");
             else if (obj is List<string>)
                 return String.Join(",", (obj as List<string>).ToArray());
diff --git a/modules/swagger-codegen/src/main/resources/csharp/ApiClient.mustache b/modules/swagger-codegen/src/main/resources/csharp/ApiClient.mustache
index cbbcf1e743f..172e6bc627a 100644
--- a/modules/swagger-codegen/src/main/resources/csharp/ApiClient.mustache
+++ b/modules/swagger-codegen/src/main/resources/csharp/ApiClient.mustache
@@ -156,6 +156,9 @@ namespace {{packageName}}.Client
         public string ParameterToString(object obj)
         {
             if (obj is DateTime)
+                // Return an ISO 8601 formatted string, also known as a Round-trip date/time pattern
+                // https://msdn.microsoft.com/en-us/library/az4se3k1(v=vs.110).aspx#Anchor_8
+                // For example: 2009-06-15T13:45:30.0000000
                 return ((DateTime)obj).ToString ("o");
             else if (obj is IList)
             {