Skip to main content
Dates in JavaScript are notoriously difficult to work with, and JSON doesn’t have a native date type. Express Zod API provides specialized schemas to handle dates correctly in both requests and responses.

​
The Date Problem

When you return a Date object in JSON, it’s automatically converted to an ISO 8601 string by calling toISOString(): 
const response = { createdAt: new Date("2022-01-22") };
JSON.stringify(response);
// {"createdAt":"2022-01-22T00:00:00.000Z"}
Similarly, dates can’t be transmitted in JSON format in their original form - they must be strings. The built-in z.date() schema doesn’t handle these conversions automatically.

​
The Solution: ez.dateIn() and ez.dateOut()

Express Zod API provides two custom schemas for handling dates:
  • ez.dateIn() - For input: accepts ISO string, validates it, transforms to Date object
  • ez.dateOut() - For output: accepts Date object, transforms to ISO string for response

​
Using ez.dateIn()

ez.dateIn() accepts ISO date strings and transforms them into JavaScript Date objects: 
import { z } from "zod";
import { ez, defaultEndpointsFactory } from "express-zod-api";

const updateUserEndpoint = defaultEndpointsFactory.build({
  method: "post",
  input: z.object({
    userId: z.string(),
    birthday: ez.dateIn({ examples: ["1963-04-21"] }),
  }),
  output: z.object({
    success: z.boolean(),
  }),
  handler: async ({ input }) => {
    // input.birthday is a Date object
    console.log(input.birthday instanceof Date); // true
    console.log(input.birthday.getFullYear()); // 1963
    
    await db.users.update({ id: input.userId, birthday: input.birthday });
    return { success: true };
  },
});

​
Supported Date Formats

ez.dateIn() accepts several ISO 8601 date/time formats: 
2021-12-31T23:59:59.000Z  // Full ISO with milliseconds and timezone
2021-12-31T23:59:59Z       // ISO without milliseconds
2021-12-31T23:59:59        // Local time without timezone
2021-12-31                 // Date only

​
Using ez.dateOut()

ez.dateOut() accepts a Date object and transforms it to an ISO string for the response: 
import { z } from "zod";
import { ez, defaultEndpointsFactory } from "express-zod-api";

const getUserEndpoint = defaultEndpointsFactory.build({
  method: "get",
  input: z.object({
    userId: z.string(),
  }),
  output: z.object({
    name: z.string(),
    createdAt: ez.dateOut({ examples: ["2021-12-31T00:00:00.000Z"] }),
  }),
  handler: async ({ input }) => {
    const user = await db.users.findById(input.userId);
    
    return {
      name: user.name,
      createdAt: user.createdAt, // Date object from database
    };
  },
});
The response will contain: 
{
  "status": "success",
  "data": {
    "name": "John Doe",
    "createdAt": "2021-12-31T00:00:00.000Z"
  }
}

​
Complete Example

Here’s a real example from the Express Zod API source code: 
import { z } from "zod";
import { ez, defaultEndpointsFactory } from "express-zod-api";

const updateUserEndpoint = defaultEndpointsFactory.build({
  method: "post",
  input: z.object({
    userId: z.string(),
    birthday: ez.dateIn({
      description: "the day of birth",
      examples: ["1963-04-21"],
    }),
  }),
  output: z.object({
    createdAt: ez.dateOut({
      description: "account creation date",
      examples: ["2021-12-31T00:00:00.000Z"],
    }),
  }),
  handler: async ({ input }) => ({
    createdAt: new Date("2022-01-22"),
  }),
});

​
Adding Metadata

Both ez.dateIn() and ez.dateOut() accept metadata that appears in generated documentation: 
ez.dateIn({
  description: "User's date of birth",
  examples: ["1990-01-15"],
})

ez.dateOut({
  description: "When the resource was created",
  examples: ["2024-03-08T12:00:00.000Z"],
})

​
Validation with Dates

You can add refinements to date schemas for additional validation: 
import { ez } from "express-zod-api";
import { z } from "zod";

const bookingEndpoint = endpointsFactory.build({
  input: z.object({
    startDate: ez.dateIn(),
    endDate: ez.dateIn(),
  })
  .refine(
    (data) => data.endDate > data.startDate,
    "End date must be after start date",
  ),
  // ...
});

​
Common Date Patterns

​
Future Dates Only

ez.dateIn().refine(
  (date) => date > new Date(),
  "Date must be in the future",
)

​
Age Verification

ez.dateIn().refine(
  (birthDate) => {
    const age = new Date().getFullYear() - birthDate.getFullYear();
    return age >= 18;
  },
  "Must be at least 18 years old",
)

​
Date Range

ez.dateIn().refine(
  (date) => {
    const now = new Date();
    const thirtyDaysAgo = new Date(now.getTime() - 30 * 24 * 60 * 60 * 1000);
    return date >= thirtyDaysAgo && date <= now;
  },
  "Date must be within the last 30 days",
)

​
Implementation Details

Here’s how ez.dateIn() works internally: 
// From express-zod-api/src/date-in-schema.ts
export const dateIn = ({ examples, ...rest } = {}) => {
  const schema = z.union([
    z.iso.date(),
    z.iso.datetime(),
    z.iso.datetime({ local: true }),
  ]);

  return schema
    .meta({ examples })
    .transform((str) => new Date(str))
    .pipe(z.date())
    .brand(ezDateInBrand as symbol)
    .meta(rest);
};
And ez.dateOut(): 
// From express-zod-api/src/date-out-schema.ts
export const dateOut = (meta = {}) =>
  z
    .date()
    .transform((date) => date.toISOString())
    .brand(ezDateOutBrand as symbol)
    .meta(meta);

​
Best Practices

Always Use ez.dateIn() for Input

Don’t use plain z.date() for input schemas. It won’t handle JSON date strings correctly.

Always Use ez.dateOut() for Output

Use ez.dateOut() to ensure consistent ISO string formatting in responses.

Store Dates as Date Objects

Work with proper Date objects in your handlers. Only convert to/from strings at API boundaries.

Include Examples

Add examples to help generate better API documentation and make your API easier to understand.

​
Troubleshooting

​
”Invalid date string” Error

Ensure the date string follows one of the supported ISO 8601 formats. Timestamps (epoch milliseconds) are not supported - convert them to ISO strings first: 
// Wrong
const timestamp = 1641000000000;

// Right  
const isoString = new Date(timestamp).toISOString();

​
Timezone Issues

All dates are handled in ISO 8601 format, which includes timezone information. Be careful when comparing dates across different timezones.

​
Next Steps

Pagination

Implement paginated endpoints

Transformations

Learn more about data transformations

Build docs developers (and LLMs) love

Get started for freeTalk to us