[BUG] [Typescript-fetch] client does not map correctly from timestamp to date

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

I have an openapi file with a server side in Java and a client side in node. Both using the openapi generator.
The server side returns the fields date-time serialized as timestimap in the json, i see that ok, the problem is that in the node client side, using typescript-fetch the dates are not deserialised correclty.
Given the timestamp 1685211548 it returns the date 1970-01-20T12:06:51.548Z

I think that is because they are mapped like this

return {
        
        'type': ShipmentTypeFromJSON(json['type']),
        'price': json['price'],
        'departureDate': (new Date(json['departureDate'])),
        'arrivalDate': (new Date(json['arrivalDate'])),
    };

In the Suggest a Fix section i clarify why i think that this is the problem.

For more info the server side is in Java 17 + springboot 3 and the date field is generated like this

import org.springframework.format.annotation.DateTimeFormat;
import java.time.OffsetDateTime;
@DateTimeFormat(iso = DateTimeFormat.ISO.DATE_TIME)
private OffsetDateTime departureDate;

and is serialized to the json as unix timestamp

openapi-generator version

6.6.0 Both client and server

OpenAPI declaration file content or url

AvaliableShipmentAirlines:
  additionalProperties: false
  type: object
  required:
    - "airlineShipmentOptions"
  properties:
    airlineShipmentOptions:
      type: array
      maxItems: 1000
      items:
        $ref: "#/components/schemas/AirlineShipmentOption"
AirlineShipmentOption:
  additionalProperties: false
  type: object
  required:
    - "airlineLogoUrl"
    - "airlineName"
    - "airlineCode"
    - "shipmentOptions"
  properties:
    airlineLogoUrl:
      maxLength: 255
      type: string
      example: https://www.somedomian.com/airlineLogo.png
    airlineName:
      maxLength: 40
      pattern: '^[A-Za-z0-9]+$'
      type: string
      example: Emirates
    airlineCode:
      maxLength: 40
      pattern: '^[A-Za-z0-9]+$'
      type: string
      example: EMX
    shipmentOptions:
        $ref: "#/components/schemas/ShipmentOptions"
ShipmentOptions:
  type: array
  maxItems: 1000
  items:
    $ref: "#/components/schemas/ShipmentOption"
ShipmentOption:
  additionalProperties: false
  type: object
  required:
    - "type"
    - "price"
    - "departureDate"
    - "arrivalDate"
  properties:
    type:
      $ref: "#/components/schemas/ShipmentType"
    price:
      type: number
      format: double
      description: Price of the shipment
      example: 1
      maximum: 999
      minimum: 0
    departureDate:
      type: string
      format: date-time
      description: Departure date of the shipment
    arrivalDate:
      type: string
      format: date-time
      description: Arrival date of the shipment

Generation Details

Client side, where the timestaps are not mapped corrreclty
package.json scripts looks like this (other names)

"scripts": {
    ...
   "clean": "rimraf ./next && rimraf .generated/*",
    "post-process": "node src/scripts/post-process.js",
    "generate-pre": "npm run clean && npx openapi-generator-cli generate --generator-key name1 name2 && npm run post-process",
    "build-name1-rest-client": "cd .generated/name1-rest-client && npm install && npm run build",
    "generate": "npm run generate-pre && npm run build-name1-rest-client && npm i .generated/name1-rest-client "
  },

openapitools.json looks like this (other names)

{
  "$schema": "./node_modules/@openapitools/openapi-generator-cli/config.schema.json",
  "spaces": 2,
  "generator-cli": {
    "version": "6.6.0",
    "storageDir": ".tmp/",
    "generators": {
      "name1": {
        "generatorName": "typescript-fetch",
        "output": ".generated/name1-rest-client",
        "glob": "node_modules/@scope/name1-openapi/open-api.{yaml,yml}",
        "additionalProperties": {
          "withInterfaces": true,
          "supportsES6": "true",
          "npmName": "name1-rest-client",
          "stringEnums": true
        }
      },
      "name2": {
        "generatorName": "typescript-fetch",
        "output": ".generated/name2-rest-client",
        "glob": "node_modules/@scope/name2-private-openapi/open-api.{yaml,yml}",
        "additionalProperties": {
          "withInterfaces": true,
          "supportsES6": "true",
          "npmName": "name2-rest-client",
          "stringEnums": true
        }
      }
    }
  }
}

Server side:

<plugin>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator-maven-plugin</artifactId>
    <version>6.6.0</version>
    <executions>
        <execution>
            <goals>
                <goal>generate</goal>
            </goals>
            <configuration>
                <inputSpec>${open-api.path}</inputSpec>
                <enablePostProcessFile>false</enablePostProcessFile>
                <library>spring-boot</library>
                <generateModels>true</generateModels>
                <generatorName>spring</generatorName>
                <generateApiDocumentation>true</generateApiDocumentation>
                <generateModelDocumentation>true</generateModelDocumentation>
                <generateApiTests>true</generateApiTests>
                <typeMappings>
                    <typeMapping>Date=OffsetDateTime</typeMapping>
                </typeMappings>
                <importMappings>Date=java.time.OffsetDateTime</importMappings>
                <configOptions>
                    <removeOperationIdPrefix>true</removeOperationIdPrefix>
                    <apiNameSuffix>DTO</apiNameSuffix>
                    <delegatePattern>true</delegatePattern>
                    <generateApi>true</generateApi>
                    <skipOperationExample>false</skipOperationExample>
                    <java17>true</java17>
                    <dateLibrary>java17</dateLibrary>
                    <groupId>com.aGreatName</groupId>
                    <basePackage>com.aGreatName</basePackage>
                    <modelPackage>com.aGreatName.model</modelPackage>
                    <apiPackage>com.aGreatName.api</apiPackage>
                    <configPackage>com.aGreatName</configPackage>
                    <configHelp>true</configHelp>
                    <interfaceOnly>true</interfaceOnly>
                    <skipDefaultInterface>false</skipDefaultInterface>
                    <useTags>true</useTags>
                </configOptions>
                <additionalProperties>
                    <additionalProperty>modelNameSuffix=DTO</additionalProperty>
                    <additionalProperty>useSpringBoot3=true</additionalProperty>
                </additionalProperties>
            </configuration>
        </execution>
    </executions>
</plugin>

Steps to reproduce

Use date-time in the openapi spec and generate client-side code using typescript fetch, then receive timestamp for that field in the json of the response.

Suggest a fix

Generate the code more like this or use a function that does the check and returns the date.

return {
        
        'type': ShipmentTypeFromJSON(json['type']),
        'price': json['price'],
        'departureDate': isNaN(json['departureDate']) ? (new Date(json['departureDate'])) : (new Date(1000 * json['departureDate'])),
        'arrivalDate': isNaN(json['arrivalDate']) ? (new Date(json['arrivalDate'])) : (new Date(1000 * json['arrivalDate'])),
    };

My workaround is this script that i apply to the output folder after generating the code and is working fine for me.

const fs = require('fs');
const path = require('path');

const generatedDir = '.generated';

function applyCustomMapping(directory) {
    const files = fs.readdirSync(directory);

    files.forEach((file) => {
        const filePath = path.join(directory, file);
        const fileStat = fs.statSync(filePath);

        if (fileStat.isDirectory()) {
            applyCustomMapping(filePath);
        } else {
            const fileContent = fs.readFileSync(filePath, 'utf8');

            // Replace "(new Date(json['fieldName']))" with "isNaN(json['fieldName']) ? (new Date(json['fieldName'])) : (new Date(1000 * json['fieldName']))"
            const modifiedContent = fileContent.replace(/\(new Date\(json\['(.*?)'\]\)\)/g, "isNaN(json['$1']) ? (new Date(json['$1'])) : (new Date(1000 * json['$1']))");
            fs.writeFileSync(filePath, modifiedContent, 'utf8');
        }
    });
}

applyCustomMapping(generatedDir);

console.log('Custom date mapping applied successfully.');

Start with the typescript-fetch model deserializer shown in the issue and the openapi-generator-cli generate entry point; the generated files are placed under .generated, with post-processing in src/scripts/post-process.js. Reproduce the 1685211548 date-time response and trace how date-time is mapped. Done means generated clients correctly deserialize the reported timestamp without the workaround.