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

[eirikb]

fix #24703

The dart generator emits this for a format: date property:

json[r'dueDate'] = _dateFormatter.format(this.dueDate!.toUtc());

_dateFormatter is DateFormat('yyyy-MM-dd'), which formats the y/m/d the DateTime already carries. It does no timezone conversion, so that .toUtc() does nothing except move the value across a day boundary before the date is read.

Coming the other way, mapDateTime parses the bare wire value "2026-09-12" with DateTime.tryParse, which gives you local midnight. Round trip in Oslo:

"2026-09-12" -> DateTime 2026-09-12 00:00 local -> toUtc() 2026-09-11 22:00Z -> "2026-09-11"

It is stable in UTC, which I bet is why this has been around so long. We only noticed because a payment deadline got printed one day early on a physical parking fine.

Removing .toUtc() should be safe in both directions. toUtc() returns this when the DateTime is already UTC, so UTC callers are unaffected, and local callers stop being shifted. The isDateTime branch right next to it keeps its .toUtc(), an instant needs a zone, a calendar date does not.

FWIW dart-dio already handles this correctly with its own Date class (plain year/month/day ints), so the two dart generators disagree about this today.

Tests

Added a DateOnlyModel fixture and two tests that between them pin all four changed template lines:

branch	covered by
plain, no pattern	requiredDate, optionalDate
plain, with pattern	patternedDate
Optional<T>, no pattern	optionalDate with useOptional=true
Optional<T>, with pattern	patternedDate with useOptional=true

Both tests fail against the old template and pass with the fix. Samples regenerated, it comes out as two lines.

Not fixed here

While I was in there I noticed format: date is also wrong in a few other places: query, header and form parameters go through parameterToString, which is type-erased and cannot tell date from date-time, path parameters use plain .toString(), and arrays of dates do not compile at all (DateTime.listFromJson does not exist). There is also an epoch-marker branch that still normalizes to UTC, and _dateFormatter has no explicit locale so it can emit non-ASCII digits if Intl.defaultLocale is set.

Those are separate defects with separate fixes and much bigger diffs, so I left them alone to keep this one small. Happy to look at them after if you want.

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (I ran the two dart configs, dart-petstore-client-lib.yaml and dart-petstore-client-lib-fake.yaml, since the template only affects dart native.)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@jaumard @amondnet @sbu-WBT @kuhnroyal @agilob @ahmednfwela

---

Summary by cubic

Stop converting Dart format: date fields to UTC before formatting. Previously we emitted _dateFormatter.format(value.toUtc()), which shifted local-midnight dates across day boundaries and serialized the wrong day; now we call _dateFormatter.format(value). The format: date-time path is unchanged and still uses .toUtc().

• Review notes

  • Update modules/openapi-generator/src/main/resources/dart2/serialization/native/native_class.mustache to remove .toUtc() in all isDate branches (plain and pattern).
  • Add tests in DartClientCodegenTest plus a DateOnlyModel fixture to cover plain/pattern and useOptional=true paths; assertions check the exact emitted lines.
  • Regenerate samples: one-line changes in format_test.dart and nullable_class.dart.

• Impact

  • No migration required for correct clients; serialized dates now match the calendar date carried by the DateTime.
  • If any code compensated for the previous off-by-one shift, remove those workarounds.

^{Written for commit e27bf96. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 5 files

_{Re-trigger cubic}