diff --git a/tests/SIGCM2.Api.Tests/Json/DateOnlyJsonConverterTests.cs b/tests/SIGCM2.Api.Tests/Json/DateOnlyJsonConverterTests.cs
new file mode 100644
index 0000000..51fb8bd
--- /dev/null
+++ b/tests/SIGCM2.Api.Tests/Json/DateOnlyJsonConverterTests.cs
@@ -0,0 +1,54 @@
+using System.Text.Json;
+using FluentAssertions;
+using SIGCM2.Api.Json;
+
+namespace SIGCM2.Api.Tests.Json;
+
+/// <summary>
+/// UDT-011 T300.30 — Unit tests for DateOnlyJsonConverter.
+/// Verifies round-trip serialization as "yyyy-MM-dd" ISO string.
+/// No DB, no HTTP.
+/// </summary>
+public sealed class DateOnlyJsonConverterTests
+{
+    private readonly JsonSerializerOptions _options = new()
+    {
+        Converters = { new DateOnlyJsonConverter() }
+    };
+
+    /// <summary>[REQ-BE-JSON-001] DateOnly serializes as "yyyy-MM-dd" string.</summary>
+    [Fact]
+    public void Serialize_DateOnly_WritesYyyyMmDdString()
+    {
+        var date = new DateOnly(2026, 5, 1);
+        var json = JsonSerializer.Serialize(date, _options);
+        json.Should().Be("\"2026-05-01\"");
+    }
+
+    /// <summary>[REQ-BE-JSON-002] "yyyy-MM-dd" string deserializes back to DateOnly.</summary>
+    [Fact]
+    public void Deserialize_YyyyMmDdString_ReturnsDateOnly()
+    {
+        var json = "\"2026-05-01\"";
+        var date = JsonSerializer.Deserialize<DateOnly>(json, _options);
+        date.Should().Be(new DateOnly(2026, 5, 1));
+    }
+
+    /// <summary>[REQ-BE-JSON-003] Invalid date format (dd/MM/yyyy) throws on deserialize.</summary>
+    [Fact]
+    public void Deserialize_InvalidFormat_ThrowsFormatException()
+    {
+        var json = "\"01/05/2026\"";  // formato dd/MM/yyyy — inválido
+        var act = () => JsonSerializer.Deserialize<DateOnly>(json, _options);
+        act.Should().Throw<FormatException>();
+    }
+
+    /// <summary>JSON null is not valid for non-nullable DateOnly — must throw.</summary>
+    [Fact]
+    public void Deserialize_Null_ThrowsException()
+    {
+        var json = "null";
+        var act = () => JsonSerializer.Deserialize<DateOnly>(json, _options);
+        act.Should().Throw<Exception>();  // JsonException desde GetString() null path
+    }
+}