diff --git a/src/Twilio/Converters/Serializers.cs b/src/Twilio/Converters/Serializers.cs
index f5360c35d..fc41432cc 100644
--- a/src/Twilio/Converters/Serializers.cs
+++ b/src/Twilio/Converters/Serializers.cs
@@ -28,8 +28,9 @@ public static string JsonObject(object input)
         public static string DateTimeIso8601(DateTime? input)
         {
             if (input == null) return null;
-
-            return input.Value.ToString("yyyy-MM-ddTHH:mm:ssZ");
+            
+            // Explicitly escape separators to avoid using system/culture defaults.
+            return input.Value.ToString("yyyy'-'MM'-'ddTHH':'mm':'ssZ");
         }
 
         public static string Url(Uri input)