Swift DateFormatter doesn't convert date according to preset Dateformat

Question

I came across an issue when I was doing an online exercise for Swfit on Exercism. However, the code I wrote haven't been able to pass the test suite provided by the website. The problem seems to lie with the date format of the resulting date object wrapped in an optional.

I was able to get the formate of incoming dateString to conform to the test suite date format. However, I was unable to make the destinationDate also conform to the test suite date format.

I tried to use the ISO8601DateFormatter, but the compiler on my old Mac doesn't suport this class. I tried my code on online Swift compilers, but the results haven't been satisfying so far, either.

The exercise is described as follows:

Calculate the moment when someone has lived for 10^9 seconds.

A gigasecond is 10^9 (1,000,000,000) seconds.

I wrote the following code:

    import Foundation
    
    func Gigasecond(from dateString: String) -> Date? {
        let GIGASECOND: Double = 1_000_000_000
        let RFC3339DateFormatter = DateFormatter()
    
        RFC3339DateFormatter.locale = Locale(identifier: "en_US_POSIX")
        RFC3339DateFormatter.dateFormat = "yyyy-MM-dd'T'hh:mm:ss"
    
        let sourceDate = RFC3339DateFormatter.date(from: dateString)
        var destinationDate: Date? = Date(timeInterval: GIGASECOND, since: sourceDate ?? Date())
        let destDateString = RFC3339DateFormatter.string(from: destinationDate ?? Date())
        destinationDate = RFC3339DateFormatter.date(from: destDateString)
      
        return destinationDate
    }

The test suite for this exercise provided by the website is as follows:

//GigasecondTests.swift

import XCTest
@testable import Gigasecond

class GigasecondTests: XCTestCase {

    func test1 () {
        let gs = Gigasecond(from: "2011-04-25T00:00:00")?.description
        XCTAssertEqual("2043-01-01T01:46:40", gs)
    }

    func test2 () {
        let gs = Gigasecond(from: "1977-06-13T00:00:00")?.description
        XCTAssertEqual("2009-02-19T01:46:40", gs)
    }

    func test3 () {
        let gs = Gigasecond(from: "1959-07-19T00:00:00")?.description
        XCTAssertEqual("1991-03-27T01:46:40", gs)
    }

    func testTimeWithSeconds () {
        let gs = Gigasecond(from: "1959-07-19T23:59:59")?.description
        XCTAssertEqual("1991-03-28T01:46:39", gs)
    }

    func testFullTimeSpecified () {
        let gs = Gigasecond(from: "2015-01-24T22:00:00")?.description
        XCTAssertEqual("2046-10-02T23:46:40", gs)
    }

    func testFullTimeWithDayRollOver () {
        let gs = Gigasecond(from: "2015-01-24T23:59:59")?.description
        XCTAssertEqual("2046-10-03T01:46:39", gs)
    }

    static var allTests: [(String, (GigasecondTests) -> () throws -> Void)] {
        return [
            ("test1 ", test1 ),
            ("test2 ", test2 ),
            ("test3 ", test3 ),
            ("testTimeWithSeconds ", testTimeWithSeconds ),
            ("testFullTimeSpecified ", testFullTimeSpecified ),
            ("testFullTimeWithDayRollOver ", testFullTimeWithDayRollOver ),
        ]
    }
} 

// LinuxMain.swift
import XCTest
@testable import GigasecondTests

XCTMain([
    testCase(GigasecondTests.allTests),
    ])

Please help see what problem there is with my code. Thank you very much!

Answer 1

You are using the wrong dateFormat hh is for 01-12 hours. What you need is HH 00-23 hours. Your dateFormat should be "yyyy-MM-dd'T'HH:mm:ss". Note that Date description will return a UTC date description (not the current timezone). You would need to use the same date formatter to generate the string from the parsed date to test for equality:

extension Formatter {
    static let rfc3339: DateFormatter = {
        let dateFormatter = DateFormatter()
        dateFormatter.locale = Locale(identifier: "en_US_POSIX")
        dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss"
        return dateFormatter
    }()
}

Your gigasecond method can be simplified as:

func gigasecond(from dateString: String) -> Date? {
    return Formatter.rfc3339.date(from: dateString)?
        .addingTimeInterval(1_000_000_000)
}

---

Testing:

if let gs = gigasecond(from: "2011-04-25T00:00:00") {
    "2043-01-01T01:46:40" == Formatter.rfc3339.string(from: gs)  // true
}

Answer 2

I made some tweaks to my code and had the luck of having it pass the test suite. My modified code is as follows:

import Foundation

func Gigasecond(from dateString: String) -> String? {
    let GIGASECOND: Double = 1_000_000_000
    let RFC3339DateFormatter = DateFormatter()

    RFC3339DateFormatter.locale = Locale(identifier: "en_US_POSIX")
    RFC3339DateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss"
    RFC3339DateFormatter.timeZone = TimeZone(secondsFromGMT: 0)

    let sourceDate = RFC3339DateFormatter.date(from: dateString)
    var destinationDate: Date? = Date(timeInterval: GIGASECOND, since: sourceDate ?? Date())
    var destDateString = RFC3339DateFormatter.string(from: destinationDate ?? Date())
    destinationDate = RFC3339DateFormatter.date(from: destDateString)
    destDateString = RFC3339DateFormatter.string(from: destinationDate ?? Date())
  
    return destDateString
}

It turned out that the resulting destinationDate needs to be reformatted with the date format "yyyy-MM-dd'T'HH:mm:ss" to obtain the required date string.

My heartfelt thanks again go to all those who have participated in the discussion about the solutions to this question. Without their advice, I wouldn't have moved towards working out a solution.

I'm expecting further discussion about more and better ways of solving the question. Thanks.

Answer 3

Here's an alternative solution to this question, which is suitable for a broader context of application. This solution is based on the suggestions of my mentor on the Exercism website.

My thanks also go to Leo Dabus who suggested that I add time zone information to the date formatter to avoid the effects of the implicitly adopted default local time zone.

The code is as follows:

import Foundation

struct Gigasecond {
    public let gigasecond : Double = 1_000_000_000
    private let rfc3339DateFormatter : DateFormatter = { 
            let myFormatter = DateFormatter()
            myFormatter.timeZone = TimeZone(identifier: "UTC")
            myFormatter.locale = Locale(identifier: "en_US_POSIX")
            myFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss"

            return myFormatter 
    }()
    let description : String

    init?(from dateString: String) {

        let sourceDate = rfc3339DateFormatter.date(from: dateString)
        guard let srcDate = sourceDate else { 
            description = "" 
            return 
        }
        let destinationDate = Date(timeInterval: gigasecond, since: srcDate)
        description = rfc3339DateFormatter.string(from: destinationDate)
    }
}

Please let me know if there's anything I can do to improve the above code. Thank you ver much.