[BUG][Kotlin] "date-time" type integrity violated when using default dateLibrary (java8)

Description

When generating the domain model using default configuration of dateLibrary (=java8 according to https://openapi-generator.tech/docs/generators/kotlin/) the generated date-time objects are represented by java.time.OffsetDateTime.
These objects are evaluated to strings that look like this:

2024-11-08T06:54:06.831187+01:00

Date times should follow the format of RFC 3339 according to https://swagger.io/docs/specification/v3_0/data-models/data-types/#strings

In other words they should look like this:

2017-07-21T17:32:28Z

Workaround: Specify another date library, such as kotlinx-datetime which represents date-times with kotlinx.datetime.Instant

openapi-generator version

7.9.0

OpenAPI declaration file content or url

        foo:
          type: string
          format: 'date-time'
          description: Example

Generation Details

Generated using Gradle plugin

Steps to reproduce

gradlew openApiGenerate

Related issues/PRs

Suggest a fix

Start by running gradlew openApiGenerate with the shown OpenAPI declaration and inspect the generated Kotlin domain model and its date-time handling. Trace how the default java8 date library represents and serializes date-time values. Done means generated values follow the stated RFC 3339 form without requiring the kotlinx-datetime workaround.