How to do it...

Let's create a function that will return a string telling us how long there is until Christmas that we can then print:

1. Define the function:

func howLongUntilChristmas() -> String {

}

2. Within the function, get the current calendar and time zone:

let calendar = Calendar.current
let timeZone = TimeZone.current

3. Get the current date and time and use the calendar to get the current year:

let now = Date()
let yearOfNextChristmas = calendar.component(.year, from: now)

4. Define date components that correspond to midnight on Christmas Day:

var components = DateComponents(calendar: calendar, 
                                timeZone: timeZone, 
                                year: yearOfNextChristmas, 
                                month: 12, 
                                day: 25, 
                                hour: 0, 
                                minute: 0, 
                                second: 0)

5. Get a Date object from those components:

var christmas = components...