NSCalendar的GMT时间不正确

Question

当我计算一天的开始时间时，会获得一个小时的时差。为什么未针对GMT + 1进行更正？

let cal = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
cal.timeZone = NSTimeZone.localTimeZone()

let startOfToday = cal.startOfDayForDate(NSDate())

print(NSTimeZone.localTimeZone())
print(startOfToday)

输出：

"Local Time Zone (Europe/Amsterdam (GMT+1) offset 3600)\n"
"2016-01-14 23:00:00 +0000\n"

Answer 1

您的代码应该可以正常工作，您不必关心时区。正如Martin R所提到的，“ NSDate是绝对时间点，没有时区”。如果确实需要使用UTC时间，则可以将calendar属性设置为UTC，以在UTC时间获取任意日期的startOfDay或正午，如下所示：

extension Calendar {
    static let utc: Calendar  = {
        var calendar = Calendar.current
        calendar.timeZone = TimeZone(identifier: "UTC")!
        return calendar
    }()
    static let localTime: Calendar  = {
        var calendar = Calendar.current
        calendar.timeZone = .current
        return calendar
    }()
}

extension Date {
    var noon: Date {
        return Calendar.localTime.date(bySettingHour: 12, minute: 0, second: 0, of: self)!
    }
    var startOfDay: Date {
        return Calendar.localTime.startOfDay(for: self)
    }
    var noonAtUTC: Date {
        return Calendar.utc.date(bySettingHour: 12, minute: 0, second: 0, of: self)!
    }
    var startOfDayAtUTC: Date {
        return Calendar.utc.startOfDay(for: self)
    }
}

print(Date().noon)               // "2018-04-30 15:00:00 +0000\n"
print(Date().startOfDay)         // "2018-04-30 03:00:00 +0000\n"

print(Date().noonAtUTC)          // "2018-04-30 12:00:00 +0000\n"
print(Date().startOfDayAtUTC)    // "2018-04-30 00:00:00 +0000\n"