fix: mark the ZodDate as string type in the OpenAPI schema

[Gamote]

This PR aims to mark the ZodDate type as string in the OpenAPI schema.

Fixes

• z.date() should return string type in openApi document #125
• z.date() generates {} in OpenAPI schema #99

What was done

In the zod-to-openapi.ts I have adde a check that will detect if the source is ZodDate, if it is the object.type will be set to string.

Additionally a test was added in the zod-to-openapi.test.ts to reflect this case.

No breaking changes were added.

[Copilot]

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 suggestion.

[Copilot]

The implementation should set the 'format' property to 'date-time' for the date type in the OpenAPI schema. Update the code to include 'object.format = "date-time"'.

Suggested change

	object.type = 'string'
	object.type = 'string'; object.format = 'date-time';

[Gamote]

@BenLorantfy I got this recommendation locally as well, but I wasn't sure that is necessary. Let me know.

[BenLorantfy]

@Gamote I don't think it's a bad idea, but it should probably be date, not date-time:

[Gamote]

@BenLorantfy In that case it might be a bit more common that people are sharing ISO strings rather than just the date. What is your take on it?

[BenLorantfy]

We might need to support both format: "date" and format: "date-time". But we need a way to tell them apart (e.g. is this zod schema a z.string().date() schema or a z.string().datetime() schema). I can look into that later tonight or tmr if we're unsure how to do so.

[Gamote]

Yeah it might be the best approach, I can try to look into it tomorrow too, but I do agree that supporting both might be the way to go.

[deweve]

Hello, if it helps, I see you can check the format of a string with isDate or isDateTime.

You can do this when you know that the schema is a string

Edit: Sorry, the library already handles the string type with date and dateTime.

If it can helps my use case of z.date is to add a transform the date object to a specific format.

This is used in a serializer.

function serializeDate(date: Date): string {
    dayjs(date).format("YYYY-MM-DD");
}

z.date().transform(serializeDate)

In this case, you cannot guess the format

You will have a ZodEffect, the zodEffect schema contains a z.Date and not a z.String.

To conclude : The recommendation is good, z.Date will be transformed in string and it is always a datetime.