🎸 Mastering Swift Calendars ⏰

Handling dates in Swift seems easy… until it isn’t. Whether it’s a daylight saving bug sneaking into production or a tricky interview…

🎸 Mastering Swift Calendars ⏰

Handling dates in Swift seems easy… until it isn’t. Whether it’s a daylight saving bug sneaking into production or a tricky interview question on Calendar.Identifier, it’s clear — dates are trickier than they look.

So why not learn it properly… using Metallica’s 2025 tour dates?

We’ll cover:

Every calendar type & locale quirk.
DateFormatter magic.
Performance tips.
Tricky interview questions.
And yeah, a live countdown view & Metallica-themed SwiftUI fun.

🎸 2025 Metallica Tour: Our Dataset

First, let’s set up the dataset we’ll be working with — a list of Metallica’s upcoming shows, including single-night and two-night events:

struct TourDate: Identifiable {  
    let id = UUID()  
    let city: String  
    let venue: String  
    let date: Date  
    let isTwoDayEvent: Bool  
}

Full dataset, including two-night and one-night shows flagged with a Bool.

let tourDates: [TourDate] = [  
    TourDate(city: "Syracuse, NY", venue: "JMA Wireless Dome", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 4, day: 19).date!, isTwoDayEvent: false),  
    TourDate(city: "Toronto, ON", venue: "Rogers Centre", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 4, day: 24).date!, isTwoDayEvent: true),  
    TourDate(city: "Toronto, ON", venue: "Rogers Centre", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 4, day: 26).date!, isTwoDayEvent: true),  
    TourDate(city: "Nashville, TN", venue: "Nissian Stadium", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 5, day: 1).date!, isTwoDayEvent: true),  
    TourDate(city: "Nashville, TN", venue: "Nissian Stadium", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 5, day: 3).date!, isTwoDayEvent: true),  
    TourDate(city: "Blacksburg, VA", venue: "Lane Stadium", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 5, day: 7).date!, isTwoDayEvent: false),  
    TourDate(city: "Columbus, OH", venue: "Historic Crew Stadium", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 5, day: 9).date!, isTwoDayEvent: true),  
    TourDate(city: "Columbus, OH", venue: "Historic Crew Stadium", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 5, day: 11).date!, isTwoDayEvent: true),  
    TourDate(city: "Philadelphia, PA", venue: "Lincoln Financial Field", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 5, day: 23).date!, isTwoDayEvent: true),  
    TourDate(city: "Philadelphia, PA", venue: "Lincoln Financial Field", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 5, day: 25).date!, isTwoDayEvent: true),  
    TourDate(city: "Washington,D.C.", venue: "Northwest Stadium", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 5, day: 28).date!, isTwoDayEvent: false),  
    TourDate(city: "Charlotte, NC", venue: "Bank of America Stadium", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 5, day: 31).date!, isTwoDayEvent: false),  
    TourDate(city: "Atlanta, GA", venue: "Mercedes-Benz Stadium", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 6, day: 3).date!, isTwoDayEvent: false),  
    TourDate(city: "Tampa, FL", venue: "Raymond James Stadium", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 6, day: 6).date!, isTwoDayEvent: true),  
    TourDate(city: "Tampa, FL", venue: "Raymond James Stadium", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 6, day: 8).date!, isTwoDayEvent: true),  
    TourDate(city: "Houston, TX", venue: "NRG Stadium", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 6, day: 14).date!, isTwoDayEvent: false),  
    TourDate(city: "Santa Clara, CA", venue: "Levi's Stadium", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 6, day: 20).date!, isTwoDayEvent: true),  
    TourDate(city: "Santa Clara, CA", venue: "Levi's Stadium", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 6, day: 22).date!, isTwoDayEvent: true),  
    TourDate(city: "Denver, CO", venue: "Empower Field at Mile High", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 6, day: 27).date!, isTwoDayEvent: true),  
    TourDate(city: "Denver, CO", venue: "Empower Field at Mile High", date: DateComponents(calendar:  Calendar(identifier: .gregorian), year: 2025, month: 6, day: 29).date!, isTwoDayEvent: true),  
]

This dataset becomes our playground for everything related to calendars, locales, and date quirks.

🗓️ Grouping Concerts by Month

We’ll create a list grouped by month/year, showing all the shows cleanly organized. Here’s how:

struct MonthSection: Identifiable {  
    let id: String    
    let title: String  
    let dates: [TourDate]  
}  
  
func groupTourDatesByMonth(dates: [TourDate]) -> [MonthSection] {  
    let grouped = Dictionary(grouping: dates) { tour in  
        let comps = Calendar(identifier: .gregorian).dateComponents([.year, .month], from: tour.date)  
        return "\(comps.year!)-\(comps.month!)"  
    }  
      
    let monthFormatter: DateFormatter = {  
        let formatter = DateFormatter()  
        formatter.dateFormat = "MMMM yyyy"  
        formatter.calendar = Calendar(identifier: .gregorian)  
        return formatter  
    }()  
      
    return grouped.map { key, datesInMonth in  
        let parts = key.split(separator: "-").map(String.init)  
        let year = Int(parts[0])!  
        let month = Int(parts[1])!  
        let date = Calendar(identifier: .gregorian).date(from: DateComponents(year: year, month: month, day: 1))!  
        let title = monthFormatter.string(from: date)  
          
        return MonthSection(id: key, title: title, dates: datesInMonth.sorted { $0.date < $1.date })  
    }.sorted { $0.id < $1.id }  
}

Simple, clean grouping using .gregorian calendar for internal consistency.

🕒 Countdown View: Days Until the Next Show

Let’s spice it up with a live countdown view. Users can even toggle between calendar systems.

struct CountdownView: View {  
    let tourDate: TourDate  
    @State private var now = Date()  
    @State private var calendarID: Calendar.Identifier = .gregorian  
      
    let timer = Timer.publish(every: 1, on: .main, in: .common).autoconnect()  
      
    var body: some View {  
        let selectedCalendar = Calendar(identifier: calendarID)  
        let components = selectedCalendar.dateComponents([.day, .hour, .minute, .second], from: now, to: tourDate.date)  
          
        VStack(spacing: 20) {  
            Text(tourDate.city).font(.largeTitle)  
            Text(tourDate.venue).font(.title2)  
              
            Text("Countdown:")  
            Text("\(components.day ?? 0)d \(components.hour ?? 0)h \(components.minute ?? 0)m \(components.second ?? 0)s")  
                .font(.headline)  
                .onReceive(timer) { now = $0 }  
              
            Picker("Calendar", selection: $calendarID) {  
                ForEach(Calendar.Identifier.allCases, id: \.self) { id in  
                    Text("\(id.rawValue)").tag(id)  
                }  
            }  
            .pickerStyle(.menu)  
        }  
        .padding()  
    }  
}

🌍 Supporting All Calendar Identifiers

Here’s a snapshot of Swift’s supported calendars:

.buddhist — Year offset (+543)

.japanese — Imperial eras, year resets with emperors

.chinese — Lunisolar, months/days don’t align with Gregorian

.hebrew — Leap months, unique year length

.islamic — Lunar, shorter year, no leap day

.persian — Week starts Saturday, used in Iran

.iso8601 — Standard for APIs, always starts week on Monday

Switching calendars mid-app? No problem — the countdown and groupings will adapt their display while the logic behind the scenes stays stable.

🎯 Formatting Dates: Locale & Calendar Magic

Want full control over how dates are displayed?

func formattedDate(for date: Date, calendarID: Calendar.Identifier, localeID: String, format: String) -> String {  
    let formatter = DateFormatter()  
    formatter.calendar = Calendar(identifier: calendarID)  
    formatter.locale = Locale(identifier: localeID)  
    formatter.dateFormat = format  
    return formatter.string(from: date)  
}

Try combining .japanese calendar with “ar_SA” locale and see what happens. Localization & cultural quirks handled like a champ.

🚨 Interview Break: Tricky Calendar Questions

Question: “How does daylight saving time affect date differences in Swift?”

Answer: If you compare two dates spanning a daylight saving time boundary, you might get unexpected results:

let calendar = Calendar(identifier: .gregorian)  
let start = Date(timeIntervalSince1970: 1700000000)   
let end = Date(timeIntervalSince1970: 1700000000 + 86_400)  
let diff = calendar.dateComponents([.hour], from: start, to: end)  
print(diff.hour!) // Could be 23 or 25, depending on DST shift

This happens because the hour count changes when clocks spring forward or fall back.

✅ The Correct Way

If you care about logical days, not exact hours, compare .day components instead of .hour:

let dayDiff = calendar.dateComponents([.day], from: start, to: end)  
print(dayDiff.day!) // Always prints 1

Or, if you want consistency regardless of time zones or DST shifts:

Set a fixed time zone (like UTC).
Avoid system defaults that depend on user settings.

var calendar = Calendar(identifier: .gregorian)  
calendar.timeZone = TimeZone(abbreviation: "UTC")!  
  
let utcDiff = calendar.dateComponents([.hour], from: start, to: end)  
print(utcDiff.hour!) // Always 24 hours difference

🚀 Performance Tips & Final Thoughts

Date handling is one of those things that looks easy until you:

Group across time zones.
Deal with non-Gregorian calendars.
Hit leap years or daylight saving shifts.

Keep it solid by:

Locking your internal logic to .gregorian.
Letting users customize display calendars/locales.
Reusing DateFormatter and Calendar instances.
Being mindful of time zone differences.

Next time an interviewer asks, “What happens when the calendar system changes mid-app?” — you’ll grin and explain how your Metallica-themed SwiftUI app handles it flawlessly.

If you’re ready to test it, tweak it, and maybe even throw in your own countdown to your favorite concert, give the demo app a spin.

More dev fun, stories, and tips? Head over to https://medium.com/@wesleymatlock and keep building apps that rock. 🤘🗓️✨

By Wesley Matlock on March 21, 2025.

Canonical link

Exported from Medium on May 10, 2025.

Written on March 21, 2025