Check how many consecutive days a user has used an app

I've already seen other questions asking about how many times the app has been opened. I want to send a local notification when the user uses the app for 31 consecutive days.

Would this be a NSUserDefaults discovery method or would I need to use an analytics API?

Use UserDefault. In appdelegate's didFinishLaunch method check for days count

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
     
    let kLastUsed = "LastUsedTime"
    let kDaysCount = "DaysCount"
    let currentDateTimeInterval = Int(Date().timeIntervalSinceReferenceDate)
    var storedDaysCount:Int = UserDefaults.standard.integer(forKey: kDaysCount)
    if storedDaysCount >= 31 {
        //show pushNotifications
    }
    else {
        let lastDateTimeInterval = UserDefaults.standard.integer(forKey: kLastUsed)
    
        let diff = currentDateTimeInterval - lastDateTimeInterval
        if diff > 86400 && diff < 172800 {
            //next day. increase day count by one
            storedDaysCount = storedDaysCount + 1
            UserDefaults.standard.set(storedDaysCount, forKey: kDaysCount)
        }
        else if diff > 86400 {
            //not next day. reset counter to 1
            UserDefaults.standard.set(1, forKey: kDaysCount)
        }
        
        UserDefaults.standard.set(currentDateTimeInterval, forKey: kLastUsed)
    }
    
    return true
}