swift 5 only the day number from date
let dateString = "2018-12-24 18:00:00 UTC"
let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd HH:mm:ss 'UTC'"
guard let date = formatter.date(from: dateString) else {
return
}
formatter.dateFormat = "yyyy"
let year = formatter.string(from: date)
formatter.dateFormat = "MM"
let month = formatter.string(from: date)
formatter.dateFormat = "dd"
let day = formatter.string(from: date)
print(year, month, day) // 2018 12 24