ISO8601 Date Formatter Benchmarks

main.swift

import AppKit

@main
struct DateFormatters {
    static func main() {
        let iterations = 100_000
        let startInterval: TimeInterval = 780_000_000
        
        time("Create Date Baseline") {
            var results: [Date] = []
            results.reserveCapacity(iterations)
            for offset in 0..<iterations {
                let date = Date(timeIntervalSinceReferenceDate: startInterval + TimeInterval(offset))
                results.append(date)
            }
            print("\(results.count) - \(results.first.map { $0.description } ?? "nil")")
        }
        
        time("Cached Date.ISO8601FormatStyle Format") {
            let formatter = Date.ISO8601FormatStyle(includingFractionalSeconds: true)
            var results: [String] = []
            results.reserveCapacity(iterations)
            for offset in 0..<iterations {
                let date = Date(timeIntervalSinceReferenceDate: startInterval + TimeInterval(offset))
                results.append(date.formatted(formatter))
            }
            print("\(results.count) - \(results.first.map { $0.description } ?? "nil")")
        }
        
        time("Lazy Date.ISO8601FormatStyle Format") {
            var results: [String] = []
            results.reserveCapacity(iterations)
            for offset in 0..<iterations {
                let date = Date(timeIntervalSinceReferenceDate: startInterval + TimeInterval(offset))
                results.append(date.formatted(.iso8601
                    .year()
                    .month()
                    .day()
                    .timeZone(separator: .omitted)
                    .time(includingFractionalSeconds: true)
                    .timeSeparator(.colon)
                ))
            }
            print("\(results.count) - \(results.first.map { $0.description } ?? "nil")")
        }
        
        time("Cached Date.ISO8601FormatStyle Parse") {
            let formatter = Date.ISO8601FormatStyle(includingFractionalSeconds: true)
            var results: [Date?] = []
            results.reserveCapacity(iterations)
            for offset in 0..<iterations {
                let date = Date(timeIntervalSinceReferenceDate: startInterval + TimeInterval(offset))
                let string = date.formatted(formatter)
                results.append(try? formatter.parse(string))
            }
            print("\(results.count) - \(results.first?.map { $0.description } ?? "nil")")
        }
        
        time("Lazy Date.ISO8601FormatStyle Parse") {
            var results: [Date?] = []
            results.reserveCapacity(iterations)
            for offset in 0..<iterations {
                let date = Date(timeIntervalSinceReferenceDate: startInterval + TimeInterval(offset))
                let string = date.formatted(.iso8601
                    .year()
                    .month()
                    .day()
                    .timeZone(separator: .omitted)
                    .time(includingFractionalSeconds: true)
                    .timeSeparator(.colon)
                )
                results.append(try? Date(string, strategy: .iso8601
                    .year()
                    .month()
                    .day()
                    .timeZone(separator: .omitted)
                    .time(includingFractionalSeconds: true)
                    .timeSeparator(.colon)
                ))
            }
            print("\(results.count) - \(results.first?.map { $0.description } ?? "nil")")
        }
        
        time("Cached ISO8601DateFormatter Format") {
            let formatter = ISO8601DateFormatter()
            formatter.timeZone = TimeZone(secondsFromGMT: 0)
            formatter.formatOptions = [
                .withInternetDateTime,
                .withDashSeparatorInDate,
                .withColonSeparatorInTime,
                .withTimeZone,
                .withFractionalSeconds
            ]
            var results: [String] = []
            results.reserveCapacity(iterations)
            for offset in 0..<iterations {
                let date = Date(timeIntervalSinceReferenceDate: startInterval + TimeInterval(offset))
                results.append(formatter.string(from: date))
            }
            print("\(results.count) - \(results.first.map { $0.description } ?? "nil")")
        }
        
        time("Lazy ISO8601DateFormatter Format") {
            var results: [String] = []
            results.reserveCapacity(iterations)
            for offset in 0..<iterations {
                let formatter = ISO8601DateFormatter()
                formatter.timeZone = TimeZone(secondsFromGMT: 0)
                formatter.formatOptions = [
                    .withInternetDateTime,
                    .withDashSeparatorInDate,
                    .withColonSeparatorInTime,
                    .withTimeZone,
                    .withFractionalSeconds
                ]
                let date = Date(timeIntervalSinceReferenceDate: startInterval + TimeInterval(offset))
                results.append(formatter.string(from: date))
            }
            print("\(results.count) - \(results.first.map { $0.description } ?? "nil")")
        }
        
        time("Cached ISO8601DateFormatter Parse") {
            let formatter = ISO8601DateFormatter()
            formatter.timeZone = TimeZone(secondsFromGMT: 0)
            formatter.formatOptions = [
                .withInternetDateTime,
                .withDashSeparatorInDate,
                .withColonSeparatorInTime,
                .withTimeZone,
                .withFractionalSeconds
            ]
            var results: [Date?] = []
            results.reserveCapacity(iterations)
            for offset in 0..<iterations {
                let date = Date(timeIntervalSinceReferenceDate: startInterval + TimeInterval(offset))
                let string = formatter.string(from: date)
                results.append(formatter.date(from: string))
            }
            print("\(results.count) - \(results.first?.map { $0.description } ?? "nil")")
        }
        
        time("Lazy ISO8601DateFormatter Parse") {
            var results: [Date?] = []
            results.reserveCapacity(iterations)
            for offset in 0..<iterations {
                let formatter = ISO8601DateFormatter()
                formatter.timeZone = TimeZone(secondsFromGMT: 0)
                formatter.formatOptions = [
                    .withInternetDateTime,
                    .withDashSeparatorInDate,
                    .withColonSeparatorInTime,
                    .withTimeZone,
                    .withFractionalSeconds
                ]
                let date = Date(timeIntervalSinceReferenceDate: startInterval + TimeInterval(offset))
                let string = formatter.string(from: date)
                results.append(formatter.date(from: string))
            }
            print("\(results.count) - \(results.first?.map { $0.description } ?? "nil")")
        }
    }
    
    static func time(_ name: String, handler: () -> ()) {
        let startTime = CACurrentMediaTime()
        handler()
        let endTime = CACurrentMediaTime()
        print("\(name) took \((endTime - startTime).formatted(.number.precision(.fractionLength(3))))s")
    }
}

Results for 100,000 iterations on an M1 Max MacBook Pro:

Test	Format Time	Parse Time
Create Date Baseline	—	0.007s
Re-used Date.ISO8601FormatStyle	0.028s	0.063s
New Date.ISO8601FormatStyle Every Time	0.053s	0.104s
Re-used ISO8601DateFormatter	0.129s	3.197s
New ISO8601DateFormatter Every Time	14.559s	17.494s

Raw Results

100000 - 2025-09-19 18:40:00 +0000
Create Date Baseline took 0.007s
100000 - 2025-09-19T18:40:00.000Z
Cached Date.ISO8601FormatStyle Format took 0.028s
100000 - 2025-09-19T18:40:00.000Z
Lazy Date.ISO8601FormatStyle Format took 0.053s
100000 - 2025-09-19 18:40:00 +0000
Cached Date.ISO8601FormatStyle Parse took 0.063s
100000 - 2025-09-19 18:40:00 +0000
Lazy Date.ISO8601FormatStyle Parse took 0.104s
100000 - 2025-09-19T18:40:00.000Z
Cached ISO8601DateFormatter Format took 0.129s
100000 - 2025-09-19T18:40:00.000Z
Lazy ISO8601DateFormatter Format took 14.559s
100000 - 2025-09-19 18:40:00 +0000
Cached ISO8601DateFormatter Parse took 3.197s
100000 - 2025-09-19 18:40:00 +0000
Lazy ISO8601DateFormatter Parse took 17.494s