demo for https://github.com/OpenAPITools/openapi-generator/pull/16732

Main.java

package org.openapitools.client;

import org.openapitools.client.api.DefaultApi;

import java.time.OffsetDateTime;

public class Main {
    public static void main(String[] args) {
        DefaultApi apiInstance = new DefaultApi(new ApiClient());
        String result = apiInstance.anythingGet(OffsetDateTime.now());
        System.out.println(result);
    }
}

spec.yaml

openapi: 3.0.3
info:
  title: Demo
  description: Demo
  version: 1.0.0
servers:
  - url: https://httpbin.org/

paths:
  /anything:
    get:
      parameters:
        - name: myParam
          in: query
          schema:
            type: string
            format: date-time
      responses:
        200:
          description: OK
          content:
            application/json:
              schema:
                type: string