[dart] Do not convert format: date to UTC before formatting

modules/openapi-generator/src/main/resources/dart2/serialization/native/native_class.mustache

@@ -82,10 +82,10 @@ class {{{classname}}} {
         {{#pattern}}
       json[r'{{{baseName}}}'] = value == null ? null : (_isEpochMarker(r'{{{pattern}}}')
         ? value.millisecondsSinceEpoch
-        : _dateFormatter.format(value.toUtc()));
+        : _dateFormatter.format(value));
         {{/pattern}}
         {{^pattern}}
-      json[r'{{{baseName}}}'] = value == null ? null : _dateFormatter.format(value.toUtc());
+      json[r'{{{baseName}}}'] = value == null ? null : _dateFormatter.format(value);
         {{/pattern}}
       {{/isDate}}
       {{^isDateTime}}
@@ -120,10 +120,10 @@ class {{{classname}}} {
       {{#pattern}}
       json[r'{{{baseName}}}'] = _isEpochMarker(r'{{{pattern}}}')
         ? this.{{{name}}}{{#isNullable}}!{{/isNullable}}{{^isNullable}}{{^required}}{{^defaultValue}}!{{/defaultValue}}{{/required}}{{/isNullable}}.millisecondsSinceEpoch
-        : _dateFormatter.format(this.{{{name}}}{{#isNullable}}!{{/isNullable}}{{^isNullable}}{{^required}}{{^defaultValue}}!{{/defaultValue}}{{/required}}{{/isNullable}}.toUtc());
+        : _dateFormatter.format(this.{{{name}}}{{#isNullable}}!{{/isNullable}}{{^isNullable}}{{^required}}{{^defaultValue}}!{{/defaultValue}}{{/required}}{{/isNullable}});
       {{/pattern}}
       {{^pattern}}
-      json[r'{{{baseName}}}'] = _dateFormatter.format(this.{{{name}}}{{#isNullable}}!{{/isNullable}}{{^isNullable}}{{^required}}{{^defaultValue}}!{{/defaultValue}}{{/required}}{{/isNullable}}.toUtc());
+      json[r'{{{baseName}}}'] = _dateFormatter.format(this.{{{name}}}{{#isNullable}}!{{/isNullable}}{{^isNullable}}{{^required}}{{^defaultValue}}!{{/defaultValue}}{{/required}}{{/isNullable}});
       {{/pattern}}
     {{/isDate}}
     {{^isDateTime}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/dart/DartClientCodegenTest.java

@@ -32,6 +32,7 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Locale;
+import java.util.Map;
 
 public class DartClientCodegenTest {
 
@@ -101,13 +102,18 @@ public void testEnumPropertyWithQuotes() {
     }
 
     private List<File> generateDartNativeFromSpec(String specPath) throws Exception {
+        return generateDartNativeFromSpec(specPath, Map.of());
+    }
+
+    private List<File> generateDartNativeFromSpec(String specPath, Map<String, Object> additionalProperties) throws Exception {
         File output = Files.createTempDirectory("dart-native-test").toFile();
         output.deleteOnExit();
 
         final CodegenConfigurator configurator = new CodegenConfigurator()
                 .setGeneratorName("dart")
                 .setInputSpec(specPath)
                 .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+        additionalProperties.forEach(configurator::addAdditionalProperty);
 
         ClientOptInput opts = configurator.toClientOptInput();
         DefaultGenerator generator = new DefaultGenerator();
@@ -199,4 +205,43 @@ public void testNullableNestedComplexArraysPreserveNullEntries() throws Exceptio
         TestUtils.assertFileContains(modelFile.toPath(),
                 "e == null ? null : NullableRequiredModel.listFromJson(e)");
     }
+
+    // DateFormat('yyyy-MM-dd') does no timezone conversion, it just formats the y/m/d the
+    // DateTime already carries. So toUtc() only shifts the value across a day boundary and
+    // the date comes out off by one - back a day east of UTC, forward a day west of it.
+    @Test(description = "format: date must not be converted to UTC before formatting")
+    public void testDateOnlyFieldsAreNotConvertedToUtc() throws Exception {
+        List<File> files = generateDartNativeFromSpec(
+                "src/test/resources/3_0/dart/dart-native-deserialization-bugs.yaml");
+
+        File modelFile = files.stream()
+                .filter(f -> f.getName().equals("date_only_model.dart"))
+                .findFirst()
+                .orElseThrow(() -> new AssertionError("date_only_model.dart not found in generated files"));
+
+        TestUtils.assertFileContains(modelFile.toPath(),
+                "json[r'requiredDate'] = _dateFormatter.format(this.requiredDate);");
+        TestUtils.assertFileContains(modelFile.toPath(),
+                "json[r'optionalDate'] = _dateFormatter.format(this.optionalDate!);");
+        // A pattern routes it through the _isEpochMarker ternary, non-epoch side formats the same way
+        TestUtils.assertFileContains(modelFile.toPath(),
+                ": _dateFormatter.format(this.patternedDate!);");
+    }
+
+    @Test(description = "format: date must not be converted to UTC in the Optional<T> path either")
+    public void testDateOnlyFieldsAreNotConvertedToUtcWithUseOptional() throws Exception {
+        List<File> files = generateDartNativeFromSpec(
+                "src/test/resources/3_0/dart/dart-native-deserialization-bugs.yaml",
+                Map.of("useOptional", true));
+
+        File modelFile = files.stream()
+                .filter(f -> f.getName().equals("date_only_model.dart"))
+                .findFirst()
+                .orElseThrow(() -> new AssertionError("date_only_model.dart not found in generated files"));
+
+        TestUtils.assertFileContains(modelFile.toPath(),
+                "json[r'optionalDate'] = value == null ? null : _dateFormatter.format(value);");
+        TestUtils.assertFileContains(modelFile.toPath(),
+                ": _dateFormatter.format(value));");
+    }
 }

modules/openapi-generator/src/test/resources/3_0/dart/dart-native-deserialization-bugs.yaml

@@ -47,3 +47,18 @@ components:
             nullable: true
             items:
               $ref: '#/components/schemas/NullableRequiredModel'
+    DateOnlyModel:
+      type: object
+      required:
+        - requiredDate
+      properties:
+        requiredDate:
+          type: string
+          format: date
+        optionalDate:
+          type: string
+          format: date
+        patternedDate:
+          type: string
+          format: date
+          pattern: '^\d{4}-\d{2}-\d{2}$'

samples/openapi3/client/petstore/dart2/petstore_client_lib_fake/lib/model/format_test.dart

@@ -233,7 +233,7 @@ class FormatTest {
     } else {
       json[r'binary'] = null;
     }
-      json[r'date'] = _dateFormatter.format(this.date.toUtc());
+      json[r'date'] = _dateFormatter.format(this.date);
     if (this.dateTime != null) {
       json[r'dateTime'] = this.dateTime!.toUtc().toIso8601String();
     } else {

samples/openapi3/client/petstore/dart2/petstore_client_lib_fake/lib/model/nullable_class.dart

@@ -108,7 +108,7 @@ class NullableClass {
       json[r'string_prop'] = null;
     }
     if (this.dateProp != null) {
-      json[r'date_prop'] = _dateFormatter.format(this.dateProp!.toUtc());
+      json[r'date_prop'] = _dateFormatter.format(this.dateProp!);
     } else {
       json[r'date_prop'] = null;
     }