diff --git a/src/api/SIGCM2.Api/Json/DateOnlyJsonConverter.cs b/src/api/SIGCM2.Api/Json/DateOnlyJsonConverter.cs
new file mode 100644
index 0000000..ca1b1e1
--- /dev/null
+++ b/src/api/SIGCM2.Api/Json/DateOnlyJsonConverter.cs
@@ -0,0 +1,31 @@
+using System.Globalization;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace SIGCM2.Api.Json;
+
+/// <summary>
+/// JSON converter for <see cref="DateOnly"/> that uses the "yyyy-MM-dd" ISO format.
+///
+/// UDT-011: Ensures Cat2 date fields (VigenciaDesde, etc.) never serialize as
+/// "2026-05-01T00:00:00" or with a UTC suffix "Z", which would mislead consumers
+/// into treating civil Argentine dates as absolute UTC instants.
+/// </summary>
+public sealed class DateOnlyJsonConverter : JsonConverter<DateOnly>
+{
+    private const string DateFormat = "yyyy-MM-dd";
+
+    public override DateOnly Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        var str = reader.GetString();
+        if (str is null)
+            throw new JsonException("DateOnly value cannot be null.");
+
+        return DateOnly.ParseExact(str, DateFormat, CultureInfo.InvariantCulture);
+    }
+
+    public override void Write(Utf8JsonWriter writer, DateOnly value, JsonSerializerOptions options)
+    {
+        writer.WriteStringValue(value.ToString(DateFormat, CultureInfo.InvariantCulture));
+    }
+}