How do i organize swift "utility" functions?

util_extensions.swift

extension String {
    func asTripDictionary() -> NSDictionary {
        return ["wat":"wat"]
    }
}

extension NSDate {
    func asLocalISOString() -> String {
        return "2015-03-12T13:24:00"
    }
}

var tripData2: NSDictionary = json.asTripDictionary()

var dateString2: String = NSDate().asLocalISOString()

utils.swift

class TripUtils {
    class func parseJSON(json: String) -> NSDictionary {
        return ["wat":"wat"]
    }
}

class DateUtils {
    class func localISOStringFromDate(date: NSDate) -> String {
        return "2015-03-12T13:24:00"
    }
}

var json = "{\"haha\": \"haha\"}"

var tripData: NSDictionary = TripUtils.parseJSON(json)

var dateString: String = DateUtils.localISOStringFromDate(NSDate())