如何从NSDateComponents获得正确的秒数？

import Foundation

func dateFromComponents(components: NSDateComponents) -> NSDate? {
    let calendar = NSCalendar.currentCalendar()
    return calendar.dateFromComponents(components)
}

func secondsBetweenDates(date1: NSDate, date2: NSDate) -> Int {
    let calendar = NSCalendar.currentCalendar()
    let components = calendar.components(.Second, fromDate: date1, toDate: date2, options: [])
    return components.second
}

let startComponents = NSDateComponents()
startComponents.year = 2021
startComponents.month = 1
startComponents.day = 1
startComponents.hour = 0
startComponents.minute = 0
startComponents.second = 0

let endComponents = NSDateComponents()
endComponents.year = 2021
endComponents.month = 1
endComponents.day = 1
endComponents.hour = 0
endComponents.minute = 1
endComponents.second = 30

if let startDate = dateFromComponents(startComponents), let endDate = dateFromComponents(endComponents) {
    let seconds = secondsBetweenDates(startDate, endDate)
    print("秒数差：\(seconds)")
} else {
    print("无法计算秒数")
}

秒数差：90