Fix MinLength incorrectly added to DateTime and Date types

When a DateTime, DateTimeOffset, DateOnly, or TimeOnly property has
[Required], it was getting MinLength=1 because these types are
represented as strings with a format in JSON Schema. This is incorrect -
MinLength should only apply to actual string properties, not date/time
types that use string representation.

Added IsDateTimeFormat check to exclude date-time, date, time, duration,
and time-span formats from the MinLength=1 logic.

Fixes #1583

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: lahma

src/NJsonSchema.Tests/Generation/AttributeGenerationTests.cs

@@ -299,5 +299,37 @@ public async Task When_dictionary_property_has_regex_attribute_then_regex_is_add
             Assert.Null(schema.Properties["Versions"].Pattern);
             Assert.NotNull(schema.Properties["Versions"].AdditionalPropertiesSchema.ActualSchema.Pattern);
         }
+
+        public class ClassWithRequiredDateTimeProperties
+        {
+            [Required]
+            public DateTime RequiredDateTime { get; set; }
+
+            [Required]
+            public DateTimeOffset RequiredDateTimeOffset { get; set; }
+
+            [Required]
+            public DateOnly RequiredDateOnly { get; set; }
+
+            [Required]
+            public TimeOnly RequiredTimeOnly { get; set; }
+
+            [Required]
+            public string RequiredString { get; set; }
+        }
+
+        [Fact]
+        public void When_required_DateTime_then_MinLength_is_not_set()
+        {
+            // Act
+            var schema = NewtonsoftJsonSchemaGenerator.FromType<ClassWithRequiredDateTimeProperties>();
+
+            // Assert
+            Assert.Null(schema.Properties["RequiredDateTime"].MinLength);
+            Assert.Null(schema.Properties["RequiredDateTimeOffset"].MinLength);
+            Assert.Null(schema.Properties["RequiredDateOnly"].MinLength);
+            Assert.Null(schema.Properties["RequiredTimeOnly"].MinLength);
+            Assert.Equal(1, schema.Properties["RequiredString"].MinLength);
+        }
     }
 }

src/NJsonSchema/Generation/JsonSchemaGenerator.cs

@@ -1205,7 +1205,8 @@ public void AddProperty(
                 if (hasRequiredAttribute &&
                     !propertyTypeDescription.IsEnum &&
                     propertyTypeDescription.Type == JsonObjectType.String &&
-                    !requiredAttribute.TryGetPropertyValue("AllowEmptyStrings", false))
+                    !requiredAttribute.TryGetPropertyValue("AllowEmptyStrings", false) &&
+                    !IsDateTimeFormat(propertyTypeDescription.Format))
                 {
                     propertySchema.MinLength = 1;
                 }
@@ -1306,6 +1307,12 @@ public bool IsPropertyIgnoredBySettings(ContextualAccessorInfo accessorInfo)
             return accessorInfo.GetAttributes(true).FirstAssignableToTypeNameOrDefault("DataMemberAttribute", TypeNameStyle.Name);
         }
 
+        private static bool IsDateTimeFormat(string? format)
+        {
+            return format is JsonFormatStrings.DateTime or JsonFormatStrings.Date or JsonFormatStrings.Time
+                or JsonFormatStrings.Duration or JsonFormatStrings.TimeSpan;
+        }
+
         private static bool HasDataContractAttribute(Type parentType)
         {
             return parentType.ToCachedType()