ISO8601FormatStyle produces unexpected results when including fractional seconds

[arthurcro]

While working on apple/swift-openapi-generator#637, I noticed ISO8601FormatStyle formattes dates with fractional seconds incorreclty. The fractional seconds field is off by one.
Also, when parsing a formatted date with fractional seconds, the result is not equal to the initial formatted date.

You can reproduce the issue with the following code snippet.

import Foundation

let initialDate = Date(timeIntervalSince1970: 1_674_036_251.123) 
let formatStyle = Date.ISO8601FormatStyle.iso8601
    .year()
    .month()
    .day()
    .time(includingFractionalSeconds: true)

let encoded = initialDate.formatted(formatStyle)
print(encoded) // prints 2023-01-18T10:04:11.122 instead of 2023-01-18T10:04:11.123
print(initialDate == (try! formatStyle.parse(encoded)) // prints false

[LordBurtz]

Hi, could not replicate, copy/pasting your snippet... on what versions are you? does the issue still exist?

$ swift --version
swift-driver version: 1.90.11.1 Apple Swift version 5.10 (swiftlang-5.10.0.13 clang-1500.3.9.4)
Target: arm64-apple-macosx14.0

$ uname -a
Darwin JPXNGP6J95 23.6.0 Darwin Kernel Version 23.6.0: Wed Jul 31 20:48:52 PDT 2024; root:xnu-10063.141.1.700.5~1/RELEASE_ARM64_T6020 arm64

[arthurcro]

Hey! Thank you for checking!
The issue still exists, I'm on the following:

$ swift --version
swift-driver version: 1.115 Apple Swift version 6.0 (swiftlang-6.0.0.9.10 clang-1600.0.26.2)
Target: arm64-apple-macosx15.0

$ uname -a
Darwin arthurs-mbp.home 24.0.0 Darwin Kernel Version 24.0.0: Mon Aug 12 20:51:54 PDT 2024; root:xnu-11215.1.10~2/RELEASE_ARM64_T6000 arm64

[jvdvleuten]

I had the same issue in my own project (not swift-openapi) when I updated my own iPhone, I first opened a thread on the Swift forums: https://forums.swift.org/t/rounding-error-in-milliseconds-using-iso8601formatstyle-instead-of-iso8601dateformatter/75206

I also opened this case on the feedback assistant: FB15418195 (Rounding error in milliseconds using ISO8601FormatStyle on iOS 18.0)

import Testing
import UIKit

struct DateFormatterTests {
    @Test func testDateEncodingDecoding() async throws {
        let originalIsoString = "2024-10-05T17:08:34.650Z"
        
        let iso8601DateFormatter = ISO8601DateFormatter()
        iso8601DateFormatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
        
        let testDate = iso8601DateFormatter.date(from: originalIsoString)!
        let oldFormatterString = iso8601DateFormatter.string(from: testDate)
        
        #expect(oldFormatterString == originalIsoString)
        
        let isoDateFormat: Date.ISO8601FormatStyle = .iso8601
            .year()
            .month()
            .day()
            .timeZone(separator: .omitted)
            .time(includingFractionalSeconds: true)
            .timeSeparator(.colon)
        
        let date = try isoDateFormat.parse(originalIsoString)
        
        let newFormatterString = isoDateFormat.format(date)
        
        #expect(newFormatterString == originalIsoString)
    }
}

It happens when using iOS 18. I have no idea how the implementation of this works, is it baked inside iOS? Could it be due to this change perhaps: 337a22e ?

[LordBurtz]

seems to be a problem of 6.0 vs 5.10 which matches the date of the change.. ill have a look over the weekend

[jvdvleuten]

@LordBurtz isn't this baked in the iOS version? How could I test this myself?

[arthurcro]

@parkera is this something you could help with when you get a chance? 🙇🏻

[parkera]

@stephentyrone was looking into this

[theoks]

We encountered the exact same issue. In some cases it's converted correctly, in others it's off by one.

Any indication when this will be fixed? It causes several issues in operations that require accuracy, especially when the value needs to be serialized and sent to a backend.

[LordBurtz]

After the last comment I thought stephentyrone was taking care of this but i'll investigate it this weekend probably