diff --git a/src/Scarlet.System.Text.Json.DateTimeConverter.Tests/DateTimeConverterResolverTests.cs b/src/Scarlet.System.Text.Json.DateTimeConverter.Tests/DateTimeConverterResolverTests.cs
new file mode 100644
index 0000000..2bb5d0e
--- /dev/null
+++ b/src/Scarlet.System.Text.Json.DateTimeConverter.Tests/DateTimeConverterResolverTests.cs
@@ -0,0 +1,150 @@
+using System.Text.Json;
+using System.Text.Json.Serialization.Metadata;
+using Scarlet.System.Text.Json.DateTimeConverter.Tests.Model;
+
+namespace Scarlet.System.Text.Json.DateTimeConverter.Tests;
+
+public class DateTimeConverterResolverTests
+{
+    [Fact]
+    public void DateTimeConverterResolver_ParameterlessConstructor_CanBeCreated()
+    {
+        // Arrange & Act
+        var resolver = new DateTimeConverterResolver();
+
+        // Assert
+        Assert.NotNull(resolver);
+    }
+
+    [Fact]
+    public void DateTimeConverterResolver_WithJsonSerializerOptionsConstructor_CanBeCreated()
+    {
+        // Arrange
+        var options = new JsonSerializerOptions();
+
+        // Act
+        var resolver = new DateTimeConverterResolver(options);
+
+        // Assert
+        Assert.NotNull(resolver);
+    }
+
+    [Fact]
+    public void DateTimeConverterResolver_ParameterlessConstructor_GetTypeInfo_ReturnsNull()
+    {
+        // Arrange
+        var resolver = new DateTimeConverterResolver();
+
+        // Act
+        // This exercises the parameterless constructor and the fallback path in GetTypeInfo(Type)
+        // since _source is null, it calls GetTypeInfo(type, Options), which returns null
+        var typeInfo = resolver.GetTypeInfo(typeof(SourceGeneratorModelWithAttribute));
+
+        // Assert
+        // Without a source resolver, GetTypeInfo should return null
+        Assert.Null(typeInfo);
+    }
+
+    [Fact]
+    public void DateTimeConverterResolver_WithOptionsConstructor_GetTypeInfo_UsesFallbackPath_ReturnsNull()
+    {
+        // Arrange
+        var options = new JsonSerializerOptions
+        {
+            TypeInfoResolver = ResolverModelJsonSerializerContext.Default
+        };
+        // Create a resolver with options - this exercises the JsonSerializerOptions constructor
+        var resolver = new DateTimeConverterResolver(options);
+
+        // Act
+        // Call the single-parameter GetTypeInfo
+        // This will use the fallback path: GetTypeInfo(type, Options) on line 59
+        // since _source is null (not a JsonSerializerContext)
+        // However, GetTypeInfo(Type, JsonSerializerOptions) needs _source to work, so it returns null
+        var typeInfo = resolver.GetTypeInfo(typeof(SourceGeneratorModelWithAttribute));
+
+        // Assert
+        // Without _source set, the resolver can't resolve type info
+        Assert.Null(typeInfo);
+    }
+
+    [Fact]
+    public void DateTimeConverterResolver_GetTypeInfo_WithJsonSerializerContextSource_UsesContextOptions()
+    {
+        // Arrange
+        var sourceContext = ResolverModelJsonSerializerContext.Default;
+        var resolver = new DateTimeConverterResolver(sourceContext);
+
+        // Act
+        // This should use the JsonSerializerContext path in GetTypeInfo(Type)
+        // where it detects _source is JsonSerializerContext and uses its Options
+        var typeInfo = resolver.GetTypeInfo(typeof(SourceGeneratorModelWithAttribute));
+
+        // Assert
+        Assert.NotNull(typeInfo);
+        Assert.Equal(typeof(SourceGeneratorModelWithAttribute), typeInfo.Type);
+    }
+
+    [Fact]
+    public void DateTimeConverterResolver_WithParameterlessConstructor_CanSerializeWithSourceContext()
+    {
+        // Arrange
+        var resolver = new DateTimeConverterResolver();
+        var options = new JsonSerializerOptions
+        {
+            TypeInfoResolver = JsonTypeInfoResolver.Combine(resolver, ResolverModelJsonSerializerContext.Default),
+            WriteIndented = true
+        };
+        var model = new SourceGeneratorModelWithAttribute
+        {
+            DateTimeProperty = new DateTime(2023, 10, 1, 12, 0, 0, DateTimeKind.Utc),
+            DateOnlyProperty = new DateOnly(2023, 10, 1)
+        };
+
+        // Act
+        var json = JsonSerializer.Serialize(model, options);
+
+        // Assert
+        // Should use default formats since resolver has no source with attributes
+        Assert.Contains("\"DateTimeProperty\": \"2023-10-01T12:00:00Z\"", json);
+        Assert.Contains("\"DateOnlyProperty\": \"2023-10-01\"", json);
+    }
+
+    [Fact]
+    public void DateTimeConverterResolver_WithOptionsConstructor_StoresOptionsForBaseClass()
+    {
+        // Arrange
+        var options = new JsonSerializerOptions
+        {
+            WriteIndented = true,
+            TypeInfoResolver = ResolverModelJsonSerializerContext.Default
+        };
+        // This tests the constructor with JsonSerializerOptions parameter
+        // The options are passed to the base class JsonSerializerContext
+        var resolver = new DateTimeConverterResolver(options);
+
+        // Act & Assert
+        // The resolver is successfully created with the options
+        // The constructor is used when the resolver is part of framework integration
+        Assert.NotNull(resolver);
+    }
+
+    [Fact]
+    public void DateTimeConverterResolver_WithIJsonTypeInfoResolverSource_UsesSourceForGetTypeInfo()
+    {
+        // Arrange
+        var sourceResolver = ResolverModelJsonSerializerContext.Default;
+        var resolver = new DateTimeConverterResolver(sourceResolver);
+        var options = new JsonSerializerOptions();
+
+        // Act
+        // Call GetTypeInfo with explicit options - this tests the two-parameter GetTypeInfo
+        var typeInfo = resolver.GetTypeInfo(typeof(SourceGeneratorModelWithAttribute), options);
+
+        // Assert
+        // Should get typeInfo from the source resolver
+        Assert.NotNull(typeInfo);
+        Assert.Equal(typeof(SourceGeneratorModelWithAttribute), typeInfo.Type);
+    }
+}
+
diff --git a/src/Scarlet.System.Text.Json.DateTimeConverter.Tests/ReflectionBasedTests.JsonDateTimeFormat.cs b/src/Scarlet.System.Text.Json.DateTimeConverter.Tests/ReflectionBasedTests.JsonDateTimeFormat.cs
new file mode 100644
index 0000000..f705a22
--- /dev/null
+++ b/src/Scarlet.System.Text.Json.DateTimeConverter.Tests/ReflectionBasedTests.JsonDateTimeFormat.cs
@@ -0,0 +1,107 @@
+using System.Text.Json;
+using Scarlet.System.Text.Json.DateTimeConverter.Tests.Model;
+
+namespace Scarlet.System.Text.Json.DateTimeConverter.Tests;
+
+public partial class ReflectionBasedTests
+{
+    [Fact]
+    public void ReflectionBased_WithoutResolver_WithFormatAttribute_DoesNotApplyFormat()
+    {
+        // Arrange
+        var options = new JsonSerializerOptions
+        {
+            WriteIndented = true
+        };
+        var originalModel = new SourceGeneratorModelWithAttribute
+        {
+            DateTimeProperty = new DateTime(2023, 10, 1, 12, 0, 0, DateTimeKind.Utc),
+            NullableDateTimeProperty = new DateTime(2023, 10, 1, 12, 0, 0, DateTimeKind.Utc),
+            DateTimeOffsetProperty = new DateTimeOffset(2023, 10, 1, 12, 0, 0, TimeSpan.Zero),
+            NullableDateTimeOffsetProperty = new DateTimeOffset(2023, 10, 1, 12, 0, 0, TimeSpan.Zero),
+            DateOnlyProperty = new DateOnly(2023, 10, 1),
+            NullableDateOnlyProperty = new DateOnly(2023, 10, 1),
+            TimeOnlyProperty = new TimeOnly(14, 30, 45),
+            NullableTimeOnlyProperty = new TimeOnly(14, 30, 45)
+        };
+        // Expected JSON with DEFAULT formats (not the custom formats from JsonDateTimeFormat attribute)
+        const string expectedJson = """
+                                    {
+                                      "DateTimeProperty": "2023-10-01T12:00:00Z",
+                                      "NullableDateTimeProperty": "2023-10-01T12:00:00Z",
+                                      "DateTimeOffsetProperty": "2023-10-01T12:00:00+00:00",
+                                      "NullableDateTimeOffsetProperty": "2023-10-01T12:00:00+00:00",
+                                      "DateOnlyProperty": "2023-10-01",
+                                      "NullableDateOnlyProperty": "2023-10-01",
+                                      "TimeOnlyProperty": "14:30:45",
+                                      "NullableTimeOnlyProperty": "14:30:45"
+                                    }
+                                    """;
+
+        // Act
+        var json = JsonSerializer.Serialize(originalModel, options);
+        var deserializedModel = JsonSerializer.Deserialize<SourceGeneratorModelWithAttribute>(json, options);
+
+        // Assert
+        Assert.NotNull(deserializedModel);
+        Assert.Equal(originalModel.DateTimeProperty, deserializedModel.DateTimeProperty);
+        Assert.Equal(originalModel.NullableDateTimeProperty, deserializedModel.NullableDateTimeProperty);
+        Assert.Equal(originalModel.DateTimeOffsetProperty, deserializedModel.DateTimeOffsetProperty);
+        Assert.Equal(originalModel.NullableDateTimeOffsetProperty, deserializedModel.NullableDateTimeOffsetProperty);
+        Assert.Equal(originalModel.DateOnlyProperty, deserializedModel.DateOnlyProperty);
+        Assert.Equal(originalModel.NullableDateOnlyProperty, deserializedModel.NullableDateOnlyProperty);
+        Assert.Equal(originalModel.TimeOnlyProperty, deserializedModel.TimeOnlyProperty);
+        Assert.Equal(originalModel.NullableTimeOnlyProperty, deserializedModel.NullableTimeOnlyProperty);
+        Assert.Equal(expectedJson, json);
+    }
+
+    [Fact]
+    public void ReflectionBased_WithoutResolver_WithFormatAttribute_WithNullValues_DoesNotApplyFormat()
+    {
+        // Arrange
+        var options = new JsonSerializerOptions
+        {
+            WriteIndented = true
+        };
+        var originalModel = new SourceGeneratorModelWithAttribute
+        {
+            DateTimeProperty = new DateTime(2023, 10, 1, 12, 0, 0, DateTimeKind.Utc),
+            NullableDateTimeProperty = null,
+            DateTimeOffsetProperty = new DateTimeOffset(2023, 10, 1, 12, 0, 0, TimeSpan.Zero),
+            NullableDateTimeOffsetProperty = null,
+            DateOnlyProperty = new DateOnly(2023, 10, 1),
+            NullableDateOnlyProperty = null,
+            TimeOnlyProperty = new TimeOnly(14, 30, 45),
+            NullableTimeOnlyProperty = null
+        };
+        // Expected JSON with DEFAULT formats (not the custom formats from JsonDateTimeFormat attribute)
+        const string expectedJson = """
+                                    {
+                                      "DateTimeProperty": "2023-10-01T12:00:00Z",
+                                      "NullableDateTimeProperty": null,
+                                      "DateTimeOffsetProperty": "2023-10-01T12:00:00+00:00",
+                                      "NullableDateTimeOffsetProperty": null,
+                                      "DateOnlyProperty": "2023-10-01",
+                                      "NullableDateOnlyProperty": null,
+                                      "TimeOnlyProperty": "14:30:45",
+                                      "NullableTimeOnlyProperty": null
+                                    }
+                                    """;
+
+        // Act
+        var json = JsonSerializer.Serialize(originalModel, options);
+        var deserializedModel = JsonSerializer.Deserialize<SourceGeneratorModelWithAttribute>(json, options);
+
+        // Assert
+        Assert.NotNull(deserializedModel);
+        Assert.Equal(originalModel.DateTimeProperty, deserializedModel.DateTimeProperty);
+        Assert.Null(deserializedModel.NullableDateTimeProperty);
+        Assert.Equal(originalModel.DateTimeOffsetProperty, deserializedModel.DateTimeOffsetProperty);
+        Assert.Null(deserializedModel.NullableDateTimeOffsetProperty);
+        Assert.Equal(originalModel.DateOnlyProperty, deserializedModel.DateOnlyProperty);
+        Assert.Null(deserializedModel.NullableDateOnlyProperty);
+        Assert.Equal(originalModel.TimeOnlyProperty, deserializedModel.TimeOnlyProperty);
+        Assert.Null(deserializedModel.NullableTimeOnlyProperty);
+        Assert.Equal(expectedJson, json);
+    }
+}
diff --git a/src/Scarlet.System.Text.Json.DateTimeConverter.Tests/SourceGeneratorBasedTests.JsonDateTimeFormat.cs b/src/Scarlet.System.Text.Json.DateTimeConverter.Tests/SourceGeneratorBasedTests.JsonDateTimeFormat.cs
index 03cf6dc..a7f22de 100644
--- a/src/Scarlet.System.Text.Json.DateTimeConverter.Tests/SourceGeneratorBasedTests.JsonDateTimeFormat.cs
+++ b/src/Scarlet.System.Text.Json.DateTimeConverter.Tests/SourceGeneratorBasedTests.JsonDateTimeFormat.cs
@@ -151,4 +151,154 @@ public void SourceGenerator_WithResolver_WithFormatAttribute_WithNullValues_Usin
         Assert.Null(deserializedModel.NullableTimeOnlyProperty);
         Assert.Equal(expectedJson, json);
     }
+
+    [Fact]
+    public void SourceGenerator_WithoutResolver_WithFormatAttribute_DoesNotApplyFormat_UsingOptions()
+    {
+        // Arrange
+        var sourceGenOptions = new JsonSerializerOptions
+        {
+            WriteIndented = true,
+            TypeInfoResolver = ResolverModelJsonSerializerContext.Default
+        };
+        var originalModel = new SourceGeneratorModelWithAttribute
+        {
+            DateTimeProperty = new DateTime(2023, 10, 1, 12, 0, 0, DateTimeKind.Utc),
+            NullableDateTimeProperty = new DateTime(2023, 10, 1, 12, 0, 0, DateTimeKind.Utc),
+            DateTimeOffsetProperty = new DateTimeOffset(2023, 10, 1, 12, 0, 0, TimeSpan.Zero),
+            NullableDateTimeOffsetProperty = new DateTimeOffset(2023, 10, 1, 12, 0, 0, TimeSpan.Zero),
+            DateOnlyProperty = new DateOnly(2023, 10, 1),
+            NullableDateOnlyProperty = new DateOnly(2023, 10, 1),
+            TimeOnlyProperty = new TimeOnly(14, 30, 45),
+            NullableTimeOnlyProperty = new TimeOnly(14, 30, 45)
+        };
+        // Expected JSON with DEFAULT formats (not the custom formats from JsonDateTimeFormat attribute)
+        const string expectedJson = """
+                                    {
+                                      "DateTimeProperty": "2023-10-01T12:00:00Z",
+                                      "NullableDateTimeProperty": "2023-10-01T12:00:00Z",
+                                      "DateTimeOffsetProperty": "2023-10-01T12:00:00+00:00",
+                                      "NullableDateTimeOffsetProperty": "2023-10-01T12:00:00+00:00",
+                                      "DateOnlyProperty": "2023-10-01",
+                                      "NullableDateOnlyProperty": "2023-10-01",
+                                      "TimeOnlyProperty": "14:30:45",
+                                      "NullableTimeOnlyProperty": "14:30:45"
+                                    }
+                                    """;
+
+        // Act
+        var json = JsonSerializer.Serialize(originalModel, sourceGenOptions);
+        var deserializedModel = JsonSerializer.Deserialize<SourceGeneratorModelWithAttribute>(json, sourceGenOptions);
+
+        // Assert
+        Assert.NotNull(deserializedModel);
+        Assert.Equal(originalModel.DateTimeProperty, deserializedModel.DateTimeProperty);
+        Assert.Equal(originalModel.NullableDateTimeProperty, deserializedModel.NullableDateTimeProperty);
+        Assert.Equal(originalModel.DateTimeOffsetProperty, deserializedModel.DateTimeOffsetProperty);
+        Assert.Equal(originalModel.NullableDateTimeOffsetProperty, deserializedModel.NullableDateTimeOffsetProperty);
+        Assert.Equal(originalModel.DateOnlyProperty, deserializedModel.DateOnlyProperty);
+        Assert.Equal(originalModel.NullableDateOnlyProperty, deserializedModel.NullableDateOnlyProperty);
+        Assert.Equal(originalModel.TimeOnlyProperty, deserializedModel.TimeOnlyProperty);
+        Assert.Equal(originalModel.NullableTimeOnlyProperty, deserializedModel.NullableTimeOnlyProperty);
+        Assert.Equal(expectedJson, json);
+    }
+
+    [Fact]
+    public void SourceGenerator_WithoutResolver_WithFormatAttribute_WithNullValues_DoesNotApplyFormat_UsingOptions()
+    {
+        // Arrange
+        var sourceGenOptions = new JsonSerializerOptions
+        {
+            WriteIndented = true,
+            TypeInfoResolver = ResolverModelJsonSerializerContext.Default
+        };
+        var originalModel = new SourceGeneratorModelWithAttribute
+        {
+            DateTimeProperty = new DateTime(2023, 10, 1, 12, 0, 0, DateTimeKind.Utc),
+            NullableDateTimeProperty = null,
+            DateTimeOffsetProperty = new DateTimeOffset(2023, 10, 1, 12, 0, 0, TimeSpan.Zero),
+            NullableDateTimeOffsetProperty = null,
+            DateOnlyProperty = new DateOnly(2023, 10, 1),
+            NullableDateOnlyProperty = null,
+            TimeOnlyProperty = new TimeOnly(14, 30, 45),
+            NullableTimeOnlyProperty = null
+        };
+        // Expected JSON with DEFAULT formats (not the custom formats from JsonDateTimeFormat attribute)
+        const string expectedJson = """
+                                    {
+                                      "DateTimeProperty": "2023-10-01T12:00:00Z",
+                                      "NullableDateTimeProperty": null,
+                                      "DateTimeOffsetProperty": "2023-10-01T12:00:00+00:00",
+                                      "NullableDateTimeOffsetProperty": null,
+                                      "DateOnlyProperty": "2023-10-01",
+                                      "NullableDateOnlyProperty": null,
+                                      "TimeOnlyProperty": "14:30:45",
+                                      "NullableTimeOnlyProperty": null
+                                    }
+                                    """;
+
+        // Act
+        var json = JsonSerializer.Serialize(originalModel, sourceGenOptions);
+        var deserializedModel = JsonSerializer.Deserialize<SourceGeneratorModelWithAttribute>(json, sourceGenOptions);
+
+        // Assert
+        Assert.NotNull(deserializedModel);
+        Assert.Equal(originalModel.DateTimeProperty, deserializedModel.DateTimeProperty);
+        Assert.Null(deserializedModel.NullableDateTimeProperty);
+        Assert.Equal(originalModel.DateTimeOffsetProperty, deserializedModel.DateTimeOffsetProperty);
+        Assert.Null(deserializedModel.NullableDateTimeOffsetProperty);
+        Assert.Equal(originalModel.DateOnlyProperty, deserializedModel.DateOnlyProperty);
+        Assert.Null(deserializedModel.NullableDateOnlyProperty);
+        Assert.Equal(originalModel.TimeOnlyProperty, deserializedModel.TimeOnlyProperty);
+        Assert.Null(deserializedModel.NullableTimeOnlyProperty);
+        Assert.Equal(expectedJson, json);
+    }
+
+    [Fact]
+    public void SourceGenerator_WithoutResolver_WithFormatAttribute_DoesNotApplyFormat_UsingContext()
+    {
+        // Arrange
+        var testModelType = typeof(SourceGeneratorModelWithAttribute);
+        var context = ResolverModelJsonSerializerContext.Default;
+        var originalModel = new SourceGeneratorModelWithAttribute
+        {
+            DateTimeProperty = new DateTime(2023, 10, 1, 12, 0, 0, DateTimeKind.Utc),
+            NullableDateTimeProperty = new DateTime(2023, 10, 1, 12, 0, 0, DateTimeKind.Utc),
+            DateTimeOffsetProperty = new DateTimeOffset(2023, 10, 1, 12, 0, 0, TimeSpan.Zero),
+            NullableDateTimeOffsetProperty = new DateTimeOffset(2023, 10, 1, 12, 0, 0, TimeSpan.Zero),
+            DateOnlyProperty = new DateOnly(2023, 10, 1),
+            NullableDateOnlyProperty = new DateOnly(2023, 10, 1),
+            TimeOnlyProperty = new TimeOnly(14, 30, 45),
+            NullableTimeOnlyProperty = new TimeOnly(14, 30, 45)
+        };
+        // Expected JSON with DEFAULT formats (not the custom formats from JsonDateTimeFormat attribute)
+        const string expectedJson = """
+                                    {
+                                      "DateTimeProperty": "2023-10-01T12:00:00Z",
+                                      "NullableDateTimeProperty": "2023-10-01T12:00:00Z",
+                                      "DateTimeOffsetProperty": "2023-10-01T12:00:00+00:00",
+                                      "NullableDateTimeOffsetProperty": "2023-10-01T12:00:00+00:00",
+                                      "DateOnlyProperty": "2023-10-01",
+                                      "NullableDateOnlyProperty": "2023-10-01",
+                                      "TimeOnlyProperty": "14:30:45",
+                                      "NullableTimeOnlyProperty": "14:30:45"
+                                    }
+                                    """;
+
+        // Act
+        var json = JsonSerializer.Serialize(originalModel, testModelType, context);
+        var deserializedModel = (SourceGeneratorModelWithAttribute?)JsonSerializer.Deserialize(json, testModelType, context);
+
+        // Assert
+        Assert.NotNull(deserializedModel);
+        Assert.Equal(originalModel.DateTimeProperty, deserializedModel.DateTimeProperty);
+        Assert.Equal(originalModel.NullableDateTimeProperty, deserializedModel.NullableDateTimeProperty);
+        Assert.Equal(originalModel.DateTimeOffsetProperty, deserializedModel.DateTimeOffsetProperty);
+        Assert.Equal(originalModel.NullableDateTimeOffsetProperty, deserializedModel.NullableDateTimeOffsetProperty);
+        Assert.Equal(originalModel.DateOnlyProperty, deserializedModel.DateOnlyProperty);
+        Assert.Equal(originalModel.NullableDateOnlyProperty, deserializedModel.NullableDateOnlyProperty);
+        Assert.Equal(originalModel.TimeOnlyProperty, deserializedModel.TimeOnlyProperty);
+        Assert.Equal(originalModel.NullableTimeOnlyProperty, deserializedModel.NullableTimeOnlyProperty);
+        Assert.Equal(expectedJson, json);
+    }
 }
\ No newline at end of file