Fix OpenAPI format for date/datetime/time/uuid/byte types across all data sources

- Add GetOpenApiFormatFromSystemType() to TypeHelper: maps CLR types + DbType to
  OpenAPI format strings (date-time, date, time, uuid, byte) per OAS 3.0 spec
- Update CreateComponentSchema and CreateSpRequestComponentSchema in OpenApiDocumentor
  to populate the 'format' field using the new helper
- Add DateOnly to _systemTypeToJsonDataTypeMap and _systemTypeToDbTypeMap so that
  PostgreSQL (Npgsql 8) and MySQL (MySqlConnector 2+) date columns work correctly
- Add DateOnly to GetEdmPrimitiveTypeFromSystemType for OData $filter support
- Add DateOnly to SchemaConverter.GetGraphQLTypeFromSystemType to prevent startup
  failures when GraphQL is enabled and date columns are present
- Add unit tests covering all per-database CLR type / DbType combinations

Author: Copilot

src/Core/Services/OpenAPI/OpenApiDocumentor.cs

@@ -1414,10 +1414,12 @@ private static OpenApiSchema CreateSpRequestComponentSchema(Dictionary<string, P
                 string parameter = kvp.Key;
                 ParameterDefinition def = kvp.Value;
                 string typeMetadata = TypeHelper.GetJsonDataTypeFromSystemType(def.SystemType).ToString().ToLower();
+                string? formatMetadata = TypeHelper.GetOpenApiFormatFromSystemType(def.SystemType, def.DbType);
 
                 properties.Add(parameter, new OpenApiSchema()
                 {
                     Type = typeMetadata,
+                    Format = formatMetadata,
                     Description = def.Description,
                     Default = def.Default is not null ? new OpenApiString(def.Default) : null
                 });
@@ -1490,6 +1492,7 @@ private static OpenApiSchema CreateComponentSchema(
                     if (dbObject.SourceDefinition.Columns.TryGetValue(backingColumnValue, out ColumnDefinition? columnDef))
                     {
                         typeMetadata = TypeHelper.GetJsonDataTypeFromSystemType(columnDef.SystemType).ToString().ToLower();
+                        formatMetadata = TypeHelper.GetOpenApiFormatFromSystemType(columnDef.SystemType, columnDef.DbType) ?? string.Empty;
                     }
 
                     if (entityConfig?.Fields != null)

src/Core/Services/TypeHelper.cs

@@ -46,6 +46,7 @@ public static class TypeHelper
             [typeof(byte[])] = DbType.Binary,
             [typeof(TimeOnly)] = DbType.Time,
             [typeof(TimeSpan)] = DbType.Time,
+            [typeof(DateOnly)] = DbType.Date,
             [typeof(object)] = DbType.Object
         };
 
@@ -77,6 +78,7 @@ public static class TypeHelper
             [typeof(TimeOnly)] = JsonDataType.String,
             [typeof(object)] = JsonDataType.Object,
             [typeof(DateTime)] = JsonDataType.String,
+            [typeof(DateOnly)] = JsonDataType.String,
             [typeof(DateTimeOffset)] = JsonDataType.String
         };
 
@@ -157,6 +159,7 @@ public static EdmPrimitiveTypeKind GetEdmPrimitiveTypeFromSystemType(Type column
                 "DateTime" => EdmPrimitiveTypeKind.DateTimeOffset,
                 "DateTimeOffset" => EdmPrimitiveTypeKind.DateTimeOffset,
                 "Date" => EdmPrimitiveTypeKind.Date,
+                "DateOnly" => EdmPrimitiveTypeKind.Date,
                 "TimeOnly" => EdmPrimitiveTypeKind.TimeOfDay,
                 "TimeSpan" => EdmPrimitiveTypeKind.TimeOfDay,
                 _ => throw new ArgumentException($"Column type" +
@@ -203,6 +206,60 @@ public static EdmPrimitiveTypeKind GetEdmPrimitiveTypeFromITypeNode(ITypeNode co
             return type;
         }
 
+        /// <summary>
+        /// Returns the OpenAPI format string for the given system type and optional DbType.
+        /// The format provides additional semantic information for string-typed fields in the OpenAPI schema.
+        /// </summary>
+        /// <param name="type">CLR type</param>
+        /// <param name="dbType">Optional DbType used to distinguish date-only (DbType.Date) from date-time types.</param>
+        /// <seealso cref="https://spec.openapis.org/oas/v3.0.1#data-types"/>
+        /// <returns>OpenAPI format string (e.g. "date-time", "date", "time", "uuid", "byte"),
+        /// or null if no standard format applies to the type.</returns>
+        public static string? GetOpenApiFormatFromSystemType(Type type, DbType? dbType = null)
+        {
+            // Resolve underlying type for nullable value types (e.g. DateTime? → DateTime).
+            Type? nullableUnderlyingType = Nullable.GetUnderlyingType(type);
+            if (nullableUnderlyingType is not null)
+            {
+                type = nullableUnderlyingType;
+            }
+
+            if (type == typeof(DateTime))
+            {
+                // DbType.Date corresponds to a date-only SQL type (e.g. SQL Server "date").
+                return dbType == DbType.Date ? "date" : "date-time";
+            }
+
+            if (type == typeof(DateOnly))
+            {
+                // DateOnly is reported by Npgsql 8 (PostgreSQL "date") and MySqlConnector 2+
+                // (MySQL "DATE") as the native CLR type for date-only columns.
+                return "date";
+            }
+
+            if (type == typeof(DateTimeOffset))
+            {
+                return "date-time";
+            }
+
+            if (type == typeof(TimeOnly) || type == typeof(TimeSpan))
+            {
+                return "time";
+            }
+
+            if (type == typeof(Guid))
+            {
+                return "uuid";
+            }
+
+            if (type == typeof(byte[]))
+            {
+                return "byte";
+            }
+
+            return null;
+        }
+
         /// <summary>
         /// Converts the .NET Framework (System/CLR) type to JsonDataType.
         /// Primitive data types in the OpenAPI standard (OAS) are based on the types supported

src/Service.GraphQLBuilder/Sql/SchemaConverter.cs

@@ -573,6 +573,7 @@ public static string GetGraphQLTypeFromSystemType(Type type)
                 "Decimal" => DECIMAL_TYPE,
                 "Boolean" => BOOLEAN_TYPE,
                 "DateTime" => DATETIME_TYPE,
+                "DateOnly" => DATETIME_TYPE,
                 "DateTimeOffset" => DATETIME_TYPE,
                 "Byte[]" => BYTEARRAY_TYPE,
                 "TimeOnly" => LOCALTIME_TYPE,

src/Service.Tests/OpenApiDocumentor/CLRtoJsonValueTypeUnitTests.cs

@@ -3,6 +3,7 @@
 #nullable enable
 using System;
 using System.Collections.Generic;
+using System.Data;
 using Azure.DataApiBuilder.Core.Services;
 using Azure.DataApiBuilder.Core.Services.OpenAPI;
 using Azure.DataApiBuilder.Service.Exceptions;
@@ -105,10 +106,70 @@ private static IEnumerable<object[]> GetTestData_SupportedSystemTypesMapToJsonVa
     [DataRow(typeof(Guid?))]
     [DataRow(typeof(TimeOnly?))]
     [DataRow(typeof(TimeSpan?))]
+    [DataRow(typeof(DateOnly?))]
     [DataTestMethod]
     public void ResolveUnderlyingTypeForNullableValueType(Type nullableType)
     {
         Assert.AreNotEqual(notExpected: JsonDataType.Undefined, actual: TypeHelper.GetJsonDataTypeFromSystemType(nullableType));
         Assert.IsNotNull(TypeHelper.GetDbTypeFromSystemType(nullableType));
     }
+
+    /// <summary>
+    /// Validates that TypeHelper.GetOpenApiFormatFromSystemType returns the expected
+    /// OpenAPI format string for each supported CLR type and DbType combination.
+    /// This covers all supported DAB data sources (MSSQL, PostgreSQL, MySQL):
+    /// - MSSQL: uses DbType.Date to distinguish SQL date from datetime/datetime2
+    /// - PostgreSQL (Npgsql 8): date columns report DateOnly as their CLR type
+    /// - MySQL (MySqlConnector 2+): date columns report DateOnly as their CLR type
+    /// </summary>
+    [DataTestMethod]
+    // date-time: DateTime without DbType (PostgreSQL timestamp, MySQL datetime/timestamp)
+    [DataRow(typeof(DateTime), null, "date-time",
+        DisplayName = "DateTime with no DbType → date-time (PostgreSQL timestamp, MySQL datetime)")]
+    // date-time: DateTime with DateTime DbType (MSSQL datetime, smalldatetime)
+    [DataRow(typeof(DateTime), DbType.DateTime, "date-time",
+        DisplayName = "DateTime with DbType.DateTime → date-time (MSSQL datetime/smalldatetime)")]
+    // date-time: DateTime with DateTime2 DbType (MSSQL datetime2)
+    [DataRow(typeof(DateTime), DbType.DateTime2, "date-time",
+        DisplayName = "DateTime with DbType.DateTime2 → date-time (MSSQL datetime2)")]
+    // date: DateTime with Date DbType (MSSQL date type)
+    [DataRow(typeof(DateTime), DbType.Date, "date",
+        DisplayName = "DateTime with DbType.Date → date (MSSQL date)")]
+    // date: DateOnly (PostgreSQL date via Npgsql 8, MySQL DATE via MySqlConnector 2+)
+    [DataRow(typeof(DateOnly), null, "date",
+        DisplayName = "DateOnly with no DbType → date (PostgreSQL date/Npgsql 8, MySQL DATE/MySqlConnector 2+)")]
+    // date: nullable DateOnly? resolves to DateOnly
+    [DataRow(typeof(DateOnly?), null, "date",
+        DisplayName = "DateOnly? → date (nullable date column)")]
+    // date-time: DateTimeOffset (MSSQL datetimeoffset, PostgreSQL timestamptz)
+    [DataRow(typeof(DateTimeOffset), null, "date-time",
+        DisplayName = "DateTimeOffset → date-time (MSSQL datetimeoffset, PostgreSQL timestamptz)")]
+    // date-time: nullable DateTime? resolves to DateTime
+    [DataRow(typeof(DateTime?), DbType.DateTime, "date-time",
+        DisplayName = "DateTime? → date-time (nullable datetime column)")]
+    // time: TimeOnly (MSSQL time, PostgreSQL time, MySQL time)
+    [DataRow(typeof(TimeOnly), null, "time",
+        DisplayName = "TimeOnly → time (MSSQL/PostgreSQL/MySQL time columns)")]
+    // time: TimeSpan maps to time as well
+    [DataRow(typeof(TimeSpan), null, "time",
+        DisplayName = "TimeSpan → time")]
+    // uuid: Guid (MSSQL uniqueidentifier, PostgreSQL uuid, MySQL char(36))
+    [DataRow(typeof(Guid), null, "uuid",
+        DisplayName = "Guid → uuid (MSSQL uniqueidentifier, PostgreSQL uuid)")]
+    // byte: byte[] (MSSQL binary/varbinary/image/timestamp, PostgreSQL bytea, MySQL binary/varbinary/blob)
+    [DataRow(typeof(byte[]), null, "byte",
+        DisplayName = "byte[] → byte (binary/bytea columns)")]
+    // No format for plain string, int, bool, etc.
+    [DataRow(typeof(string), null, null,
+        DisplayName = "string → null (no format)")]
+    [DataRow(typeof(int), null, null,
+        DisplayName = "int → null (no format)")]
+    [DataRow(typeof(bool), null, null,
+        DisplayName = "bool → null (no format)")]
+    public void GetOpenApiFormatFromSystemType_ReturnsExpectedFormat(Type type, DbType? dbType, string? expectedFormat)
+    {
+        string? actualFormat = TypeHelper.GetOpenApiFormatFromSystemType(type, dbType);
+        Assert.AreEqual(expectedFormat, actualFormat,
+            $"Expected OpenAPI format '{expectedFormat ?? "null"}' for type {type.Name} with DbType {dbType?.ToString() ?? "null"}.");
+    }
 }