fix(dynamo-gen): use robust DateTime check in UnixTimestampTypeHandler

src/Clients/Goa.Clients.Dynamo.Generator/Attributes/AttributeHandlerRegistry.cs

@@ -61,26 +61,12 @@ private static bool IsAttributeApplicable(AttributeInfo attributeInfo, ISymbol s
                 type = named.TypeArguments[0];
             }
 
-            return IsDateTimeType(type);
+            return type.IsDateTimeOrDateTimeOffset();
         }
 
         return true;
     }
 
-    /// <summary>
-    /// Determines whether the given type is System.DateTime or System.DateTimeOffset
-    /// using robust symbol identity rather than brittle name comparisons.
-    /// </summary>
-    private static bool IsDateTimeType(ITypeSymbol type)
-    {
-        if (type.SpecialType == SpecialType.System_DateTime)
-            return true;
-
-        // DateTimeOffset has no SpecialType, so identify it by its fully-qualified name.
-        return type.Name == nameof(DateTimeOffset)
-               && type.ContainingNamespace?.ToDisplayString() == "System";
-    }
-
     /// <summary>
     /// Validates all attributes on a symbol and reports diagnostics.
     /// </summary>

src/Clients/Goa.Clients.Dynamo.Generator/Models/TypeSymbolExtensions.cs

@@ -0,0 +1,23 @@
+using Microsoft.CodeAnalysis;
+
+namespace Goa.Clients.Dynamo.Generator.Models;
+
+/// <summary>
+/// Helper extensions for inspecting Roslyn type symbols.
+/// </summary>
+internal static class TypeSymbolExtensions
+{
+    /// <summary>
+    /// Determines whether the given type is System.DateTime or System.DateTimeOffset
+    /// using robust symbol identity rather than brittle name comparisons.
+    /// </summary>
+    public static bool IsDateTimeOrDateTimeOffset(this ITypeSymbol type)
+    {
+        if (type.SpecialType == SpecialType.System_DateTime)
+            return true;
+
+        // DateTimeOffset has no SpecialType, so identify it by its fully-qualified name.
+        return type.Name == nameof(DateTimeOffset)
+               && type.ContainingNamespace?.ToDisplayString() == "System";
+    }
+}

src/Clients/Goa.Clients.Dynamo.Generator/TypeHandlers/UnixTimestampTypeHandler.cs

@@ -13,9 +13,8 @@ public bool CanHandle(PropertyInfo propertyInfo)
     {
         var underlyingType = propertyInfo.UnderlyingType;
         var hasUnixTimestamp = propertyInfo.Attributes.Any(a => a is UnixTimestampAttributeInfo);
-
-        return hasUnixTimestamp && 
-               (underlyingType.Name == nameof(DateTime) || underlyingType.Name == nameof(DateTimeOffset));
+
+        return hasUnixTimestamp && underlyingType.IsDateTimeOrDateTimeOffset();
     }
 
     public string GenerateToAttributeValue(PropertyInfo propertyInfo)

tests/Clients/Goa.Clients.Dynamo.Generator.Tests/TypeHandlers/UnixTimestampTypeHandlerTests.cs

@@ -1,3 +1,4 @@
+using Microsoft.CodeAnalysis;
 using Goa.Clients.Dynamo.Generator.TypeHandlers;
 using Goa.Clients.Dynamo.Generator.Tests.Helpers;
 
@@ -92,6 +93,33 @@ await Assert.That(result)
             .IsFalse();
     }
 
+    [Test]
+    public async Task CanHandle_WithUserDefinedTypeNamedDateTime_ShouldReturnFalse()
+    {
+        // Arrange: a user-defined type named "DateTime" outside the System namespace
+        // must not be treated as System.DateTime. The robust symbol-based check relies
+        // on SpecialType/namespace rather than the type name alone.
+        var spoofedDateTime = MockSymbolFactory.CreateNamedTypeSymbol(
+            "DateTime",
+            "MyApp.DateTime",
+            namespaceName: "MyApp",
+            specialType: SpecialType.None).Object;
+
+        var propertyInfo = TestModelBuilders.CreateUnixTimestampPropertyInfo(
+            "FakeTimestamp",
+            spoofedDateTime,
+            isNullable: false,
+            format: Models.UnixTimestampFormat.Seconds
+        );
+
+        // Act
+        var result = _handler.CanHandle(propertyInfo);
+
+        // Assert
+        await Assert.That(result)
+            .IsFalse();
+    }
+
     [Test]
     public async Task GenerateToAttributeValue_NonNullableDateTime_SecondsFormat_ShouldGenerateCorrectCode()
     {