How to decode multiple date formats (ISO8601 string, Unix timestamp, null) into a single Date? using a Swift property wrapper [closed]

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I have a Swift model using Codable, and my backend is inconsistent with date formats:

Sometimes a date comes as ISO8601 string ("2025-09-30T04:00:00Z")

Sometimes as a Unix timestamp (1696036800)

Sometimes as null.

Right now, I’m repeating custom decoding logic in each model:

struct Event: Codable {
    let createdAt: Date?

    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        if let string = try? container.decode(String.self),
           let date = ISO8601DateFormatter().date(from: string) {
            createdAt = date
        } else if let timestamp = try? container.decode(Double.self) {
            createdAt = Date(timeIntervalSince1970: timestamp)
        } else {
            createdAt = nil
        }
    }
}

This works, but it duplicates code across models.

Question: How can I move this decoding logic into a @propertyWrapper, so I can reuse it like this?

struct Event: Codable {
    @FlexibleDate var createdAt: Date?
}

The real solution is to fix the backend. The standard date representation in JSON is ISO8601. All JSON serializers will handle ISO8601. None will treat raw integers as second-based Unix timestamps (who says the number isn't milliseconds?). Around 2000 Microsoft used the /Date(1224043200000)/ format to pass Unix timestamps as unambiguous text, at a time when even JavaScript didn't support ISO8601 properly, but by 2012 most people used ISO8601, and by 2015 this became the actual standard. 1696036800 is an unusual value — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Oct 1 at 13:54
If you which formats you need to handle you may check the string against regular expressions before trying to make the conversion. — Ptit Xav
– Ptit Xav, Commented Oct 1 at 14:02
@JoakimDanielson, thanks for pointing this out. I’ll edit the question to clarify the specific problem I was facing. — Muaaz Ahmed
– Muaaz Ahmed, Commented Oct 2 at 6:01

Sweeper · Accepted Answer · 2025-10-02 01:06:27Z

Your current code is already very close to a property wrapper. All you need to do is rename Event to FlexibleDate, and createdAt to wrappedValue, and finally add the @propertyWrapper attribute.

@propertyWrapper
struct FlexibleDate: Codable {
    let wrappedValue: Date?

    init(from decoder: any Decoder) throws {
        let container = try decoder.singleValueContainer()
        if let string = try? container.decode(String.self),
           let date = try? Date.ISO8601FormatStyle().parse(string) {
            wrappedValue = date
        } else if let timestamp = try? container.decode(Double.self) {
            wrappedValue = Date(timeIntervalSince1970: timestamp)
        } else {
            wrappedValue = nil
        }
    }
}

Note that I have also changed to use Date.ISO8601FormatStyle instead of ISO8601DateFormatter, so that you don't unnecessarily allocate memory for the date formatter every time this is decoded.

Leo Dabus · Accepted Answer · 2025-10-03 00:33:44Z

You should never throw away the errors specially when decoding JSON. Just catch the error and try again. It will also allow you to declare your property non optional:

@propertyWrapper
struct FlexibleDate: Codable {
    let wrappedValue: Date
    public init(from decoder: any Decoder) throws {
        let container = try decoder.singleValueContainer()
        do {
            wrappedValue = Date(timeIntervalSince1970: try container.decode(Double.self))
        } catch let DecodingError.typeMismatch(type, _) where type == Double.self {
            wrappedValue = try Date(container.decode(String.self), strategy: .iso8601)
        }
    }
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(wrappedValue)
    }
}

If you need an optional wrapper as well you can do as follow:

@propertyWrapper
struct OptionalFlexibleDate: Codable {
    let wrappedValue: Date?
    public init(from decoder: any Decoder) throws {
        self.wrappedValue = try decoder.singleValueContainer().decodeNil() ? 
            nil : FlexibleDate(from: decoder).wrappedValue
    }
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(wrappedValue)
    }
}

Usage:

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        do {
//            let encoder = JSONEncoder()
//            encoder.dateEncodingStrategy = .iso8601  // or .secondsSince1970
//            let jsonData = try encoder.encode(AFlexibleDate(createdAt: .now, modifiedAt: .now))
            let jsonString = #" {"createdAt":"2025-10-02T17:25:23Z","modifiedAt":1759425923.0}"#
            let jsonData = Data(jsonString.utf8)
            let event = try JSONDecoder().decode(AFlexibleDate.self, from: jsonData)
            print(event.createdAt)  // 2025-10-02 17:25:23 +0000
            print(event.modifiedAt ?? "nil") // 2025-10-02 17:25:23 +0000
        } catch {
            print(error)
        }
    }
}

struct AFlexibleDate: Codable {
    @FlexibleDate var createdAt: Date
    @OptionalFlexibleDate var modifiedAt: Date?
}

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