Swift 日期比較運算子 <、==、>

extension NSDate: Comparable { }

public func ==(lhs: NSDate, rhs: NSDate) -> Bool {
    return lhs === rhs || lhs.compare(rhs) == .OrderedSame
}

public func <(lhs: NSDate, rhs: NSDate) -> Bool {
    return lhs.compare(rhs) == .OrderedAscending
}

public func >(lhs: NSDate, rhs: NSDate) -> Bool {
    return lhs.compare(rhs) == .OrderedDescending
}

// true
NSDate() < NSDate()
// false
NSDate() == NSDate()
// false
NSDate() > NSDate()