SwiftDateExtension 是 Swift 日期扩展。
简单示例：
//-- Adding time uNits:
let date1: NSDate = NSDate().plUsmonths(2).minusDays(5).plusSeconds(5)
let date2: NSDate = NSDate().plusYears(1).minusMonths(6)
//-- Calculating difference between dates:
let weeksBetween = NSDate.monthsBetween(date1: someDate, date2: someOtherDate)
let monthsBetween = NSDate.weeksBetween(date1: someDate, date2: someOtherDate)