Creating UILocalNotifications with CoreData attribute - swift

Question

In my iOS app I am trying to create a localNotification to notify the user 15 minutes prior to the event beginning. However I am stuck. I am using CoreData to store data. I have an Appointment object which can be created. A date attribute is associated with a Appointment object. I am really stuck with it. I do not know how to set up the timeInterval and the rest of the notification process.
I do not know how to set up the timeInterval from the time the Appointment is created to 15 minutes prior to when it begins.
Here is some of my code:
func scheduleNotifications() {
let content = UNMutableNotificationContent()
guard let client = client, let name = client.name, let formula = formula, let date = formula.date else { return }
content.title = "BookMe"
content.subtitle = ""
content.body = "Your appointment with \(name) will begin soon."
content.badge = 1
let trigger = UNTimeIntervalNotificationTrigger(timeInterval: ??, repeats: false)
Edited: This is what I have but nothing is firing.
let date = formula.date
let fireDate = Calendar.current.date(byAdding: DateComponents(minute: -15), to: date as Date)
guard let timeInterval = fireDate?.timeIntervalSince(Date()) else { return }
let trigger = UNTimeIntervalNotificationTrigger(timeInterval: timeInterval, repeats: false)
let request = UNNotificationRequest(identifier: self.timerUserNotificationIdentifier, content: content, trigger: trigger)
UNUserNotificationCenter.current().add(request, withCompletionHandler: nil)

Answer 1

As I understand it you are looking to find a time interval between right now and 15 minutes before some date so that you can fire a notification 15 minutes before that date.
Here's a quick example I knocked up in a playground
// Create a date in the future - this is what you get from your own data object and I'm creating it here just so I have a date.
let scheduledDate = Calendar.current.date(from: DateComponents(year: 2017, month: 09, day: 22, hour: 22))!
// Create a date 15 minutes earlier than your shcheduled date, this is when you want your notification to fire
let fireDate = Calendar.current.date(byAdding: DateComponents(minute: -15), to: scheduledDate)!
// Then just work out the time interval between the fire date and now to get the time interval
let timeInterval = fireDate.timeIntervalSince(Date())
Excuse the force unwrapping of the created dates, these are because it's an example, you should instead not use exclamation marks and handle errors gracefully.
edited to add
UNTimeIntervalNotificationTrigger, which you are trying to use requires a TimeInterval between now and the time you want to fire the notification. A TimeInterval is a Double that represents a number of seconds. In some cases, such as this one, it represents a delay, the number of seconds between now and the time you want to fire the the notification. In other cases it represents a date by the number of seconds from a fixed date. This fixed date is either timeIntervalSince1970 - "The interval between the date object and 00:00:00 UTC on 1 January 1970." Which is what you use for UNIX timestamps or timeIntervalSinceReferenceDate - "The interval between the date object and 00:00:00 UTC on 1 January 2001."
Whatever you do, resist the temptation to modify dates by adding or removing numbers of seconds directly, use DateComponents instead.