Swift version of Averages/Mean time of day (https://rosettacode.org/wiki/Averages/Mean_time_of_day)

circularMean.swift

func circularMean() {
  let formatter = DateFormatter()
  formatter.dateFormat = "yyyy/MM/dd HH:mm:ss zzz"
  let times = [Date](arrayLiteral: formatter.date(from: "2017/10/17 23:00:17 UTC")!, formatter.date(from: "2017/10/17 23:40:20 UTC")!, formatter.date(from: "2017/10/17 00:12:45 UTC")!, formatter.date(from: "2017/10/17 00:17:19 UTC")!)
  // 1 second of time = 360/(24 * 3600) = 1/240th degree
  let angles = times.map { secondOfDay(fromDate: $0) / 240 }
  print(degreeToTime(degree: meanAngle(angles: angles), forDate: Date())!)
}

func meanAngle(angles: [Double]) -> Double {
  let sinSum = angles.reduce(0) {  sin($1 * Double.pi / 180) + $0 }
  let cosSum = angles.reduce(0) {  cos($1 * Double.pi / 180) + $0 }
  return atan2(sinSum / Double(angles.count), cosSum / Double(angles.count)) * 180 / Double.pi
}

func degreeToTime(degree: Double, forDate date: Date) -> Date? {
  var dd = degree
  if dd < 0.0 {
    dd += 360.0
  }
  var seconds  = Int(dd * 240.0)
  let hours = seconds / 3600
  var minutes  = seconds % 3600
  seconds  = minutes % 60
  minutes /= 60

  let calendar = Calendar.current

  // Specify date components
  var dateComponents = DateComponents()
  dateComponents.year = calendar.component(.year, from: date)
  dateComponents.month = calendar.component(.month, from: date)
  dateComponents.day = calendar.component(.day, from: date)
  dateComponents.timeZone = TimeZone(abbreviation: "UTC")
  dateComponents.hour = hours
  dateComponents.minute = minutes
  dateComponents.second = seconds

  return calendar.date(from: dateComponents)
}

func secondOfDay(fromDate date: Date) -> Double {
  var calendar = Calendar(identifier: .iso8601)
  calendar.timeZone = TimeZone(abbreviation: "UTC")
  let startOfDay = calendar.startOfDay(for: date)
  return date.timeIntervalSince(startOfDay)
}

circularMean()