Testing Time-Dependent Code

Your subscription expiration check passes today. Tomorrow it fails. Next month it fails differently.

Follow along with the code: iOS-Practice on GitHub

The Problem with Date()

class SubscriptionManagerBefore {
    func isSubscriptionValid(_ subscription: Subscription) -> Bool {
        // ❌ Can't control "now" in tests
        return subscription.expirationDate > Date()
    }

    func daysUntilExpiration(_ subscription: Subscription) -> Int {
        // ❌ Will give different results at different times
        let calendar = Calendar.current
        let components = calendar.dateComponents(
            [.day],
            from: Date(),
            to: subscription.expirationDate
        )
        return max(0, components.day ?? 0)
    }

    func shouldShowRenewalReminder(_ subscription: Subscription) -> Bool {
        // ❌ Can't test the 7-day threshold
        let daysLeft = daysUntilExpiration(subscription)
        return daysLeft <= 7 && daysLeft > 0
    }
}

Want to try it yourself? In DateTimeExerciseView.swift, delete everything from line 96 onwards (the // MARK: - AFTER section) and try to refactor SubscriptionManagerBefore to be testable. Your goal: make all tests in DateTimeExerciseTests.swift pass.

Why These Tests Fail

Tests using Date() directly are inherently flaky:

• Midnight boundary: Test passes at 11:59 PM, fails at 12:01 AM
• Month/year boundaries: Edge cases around Feb 28/29, Dec 31
• Time zones: Runs fine locally, fails in CI (different timezone)
• Race conditions: Test runs slower one day, date changes mid-test

You can't test "expires in 7 days" logic if you can't control what "now" means.

The Pattern: Inject Time

Instead of calling Date() directly, inject a date provider:

protocol DateProviding {
    var now: Date { get }
}

struct SystemDateProvider: DateProviding {
    var now: Date { Date() }
}

Then use it in your code:

class SubscriptionManager {
    private let dateProvider: DateProviding

    init(dateProvider: DateProviding = SystemDateProvider()) {
        self.dateProvider = dateProvider
    }

    func isSubscriptionValid(_ subscription: Subscription) -> Bool {
        return subscription.expirationDate > dateProvider.now
    }

    func daysUntilExpiration(_ subscription: Subscription) -> Int {
        let calendar = Calendar.current
        let components = calendar.dateComponents(
            [.day],
            from: calendar.startOfDay(for: dateProvider.now),
            to: calendar.startOfDay(for: subscription.expirationDate)
        )
        return max(0, components.day ?? 0)
    }
}

Production code uses SystemDateProvider() by default. Tests inject a mock.

Common Time-Dependent Bugs

The fix is always the same: make time a dependency you control.