Find time zones where it's currently a certain time

For a project I’m working I needed a function that returns the time zones where it’s currently 9am. I generalized the function to be able to find time zones where it’s currently any time. My Swift implementation was inspired by this stackoverflow answer, providing code for the same problem in Ruby.

func timeZonesWhereItIs(hour: Int, _ minute: Int = 0) -> [NSTimeZone] {
  let calendar = NSCalendar.init(calendarIdentifier: NSCalendarIdentifierGregorian)!
  calendar.timeZone = NSTimeZone(name: "UTC")!
  let currentUTCTime = NSDate()

  return NSTimeZone.knownTimeZoneNames().flatMap(NSTimeZone.init).filter { timeZone in
      let components = calendar.componentsInTimeZone(timeZone, fromDate: NSDate())
      components.hour = hour
      components.minute = minute
      let date = calendar.dateFromComponents(components)!

      return calendar.isDate(date, equalToDate: currentUTCTime, toUnitGranularity: .Hour)
  }
}

Usage:

timeZonesWhereItIs(12, 14)

To find a time zone at each hour offset use it as follows:

func timesZonesForEveryHour() -> [NSTimeZone] {
  return (0..<24).flatMap { timeZonesWhereItIs($0).first }
}

Note that there are time zones that are offset by 30 and 15 minutes. This function won’t return those.

Have fun.