NSDate在月份的第一天返回错误的日期

import Foundation

func getFirstDayOfMonth(date: NSDate) -> NSDate {
    let calendar = NSCalendar.currentCalendar()
    let components = calendar.components([.Year, .Month], fromDate: date)
    let firstDayOfMonth = calendar.dateFromComponents(components)!
    return firstDayOfMonth
}

let date = NSDate()
let firstDayOfMonth = getFirstDayOfMonth(date)
print("First day of the month: \(firstDayOfMonth)")