How do I implement a custom Swift Date type with a 'yyyy-MM-dd' initializer?

Customizing the Codable behaviour on a per-field basis is best done with a property wrapper.

This is nearly identical to Joakim's answer, except the struct needs to be tagged @propertyWrapper, and its variable has to be called wrappedValue instead of date.

The key difference is that @DateOnly var d2: Date is still just a Date, just like var d1: Date. You can access it directly, without need to access some child field like .date.

SWIFTCopied
import Foundation

// A wrapper which encodes or decodes using a custom yyyy-mm-dd format.
@propertyWrapper
struct DateOnly: Codable {
    let wrappedValue: Date
    
    private static let formatStyle: Date.FormatStyle =
        .dateTime
            .year(.defaultDigits)
            .month(.twoDigits)
            .day(.twoDigits)

    init(wrappedValue: Date) {
        self.wrappedValue = wrappedValue
    }

    init(from decoder: any Decoder) throws {
        let container = try decoder.singleValueContainer()
        let string = try container.decode(String.self)
        self.wrappedValue = try Self.formatStyle.parse(string)
    }

    func encode(to encoder: any Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.wrappedValue.formatted(Self.formatStyle))
    }
}

Example usage:

SWIFTCopied
struct S: Codable {
    var d1: Date
    @DateOnly var d2: Date
}

let exampleStruct = S(d1: Date(), d2: Date())

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted

let json = String(data: try encoder.encode(exampleStruct), encoding: .utf8)!
print(json)
// {
//  "d1" : 759528257.233482,
//  "d2" : "2025-01-25"
//}

What you need is to adjust your custom JSONDecoder.DateDecodingStrategy:

iOS 15 or later

SWIFTCopied
extension ParseStrategy where Self == Date.ISO8601FormatStyle {
    static var iso8601withFractionalSeconds: Self { .init(includingFractionalSeconds: true) }
    static var iso8601withoutTime: Self { iso8601.year().month().day() }
}

SWIFTCopied
extension JSONDecoder.DateDecodingStrategy {
    static let iso8601withOptionalTime = custom {
        let string = try $0.singleValueContainer().decode(String.self)
        do {
            return try .init(string, strategy: .iso8601withFractionalSeconds)
        } catch {
            return try .init(string, strategy: .iso8601withoutTime)
        }
    }
}

Playground testing

SWIFTCopied
struct ISOWithOpionalTimeDates: Codable {
    let dateWithFractionalSeconds: Date
    let dateWithoutTime: Date
}

let isoDatesJSON = """
{
"dateWithFractionalSeconds": "2017-06-19T18:43:19.123Z",
"dateWithoutTime": "2017-06-19",
}
"""

let isoDatesData = Data(isoDatesJSON.utf8)

let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601withOptionalTime

do {
    let isoDates = try decoder.decode(ISOWithOpionalTimeDates.self, from: isoDatesData)
    print(isoDates)
} catch {
    print(error)
}

This will print

ISOWithOpionalTimeDates(dateWithFractionalSeconds: 2017-06-19 18:43:19 +0000, dateWithoutTime: 2017-06-19 00:00:00 +0000)

If you need to support older than iOS15 you can check this post

Rather than trying to subclass a Foundation type I believe it is better to wrap it in a custom type and add custom encoding and decoding to that type.

SWIFTCopied
struct DateOnly: Codable {
    let date: Date
    private static let formatStyle: Date.FormatStyle =
        .dateTime
            .year(.defaultDigits)
            .month(.twoDigits)
            .day(.twoDigits)

    init(dateString: String) throws {
        self.date = try Self.formatStyle.parse(dateString)
    }

    init(date: Date) {
        self.date = date
    }

    init(from decoder: any Decoder) throws {
        let container = try decoder.singleValueContainer()
        let string = try container.decode(String.self)
        self.date = try Self.formatStyle.parse(string)
    }

    func encode(to encoder: any Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.date.formatted(Self.formatStyle))
    }
}