Ios NSCalendar获取日期组件有问题

所以我有一个NSCalendar并做了一些测试：

首先，将系统时区更改为

America/Los_Angeles（GMT-8）

，给出unix时间戳

145555200000

，将其转换为NSDate:by

[NSDate dateWithTimeIntervalSince1970:1455552000000 / 1000];

它返回

2016-02-15 16:00:00+0000

然后使用日历生成日期组件：

+(void)applyCalendarToCurrentTimeZone:(NSCalendar *)calendar {
    if (![calendar.timeZone isEqualToTimeZone:[NSTimeZone localTimeZone]]) {
        calendar.timeZone = [NSTimeZone localTimeZone];
    }
}

+ (NSDateComponents *)getDateComponentsFromDate:(NSDate *)date {
    NSDateComponents *components;
    static NSCalendar *ISO8601 = nil;
    static NSCalendarUnit unit;
    if (!ISO8601) {
        ISO8601 = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierISO8601];
        ISO8601.firstWeekday = 2; // Sunday = 1, Saturday = 7
        ISO8601.minimumDaysInFirstWeek = 7;
        unit = NSCalendarUnitYear|NSCalendarUnitYearForWeekOfYear|NSCalendarUnitQuarter|NSCalendarUnitWeekOfYear|NSCalendarUnitMonth|NSCalendarUnitDay|NSCalendarUnitHour|NSCalendarUnitMinute|NSCalendarUnitSecond;
    }

    [ChartModel applyCalendarToCurrentTimeZone:ISO8601];

    components = [ISO8601 components:unit fromDate:date];
    return components;
}

但在调试时，我发现了问题

首先，一切看起来都很好，日期组件是正确的，

Day

值在GMT-8时区为

15

(lldb) po ISO8601.timeZone
America/Los_Angeles (GMT-8) offset -28800

(lldb) po date
2016-01-18 16:00:00 +0000

(lldb) po ISO8601.timeZone
America/Los_Angeles (GMT-8) offset -28800

(lldb) po date
2016-02-15 16:00:00 +0000

(lldb) po [ISO8601 components:unit fromDate:date];
<NSDateComponents: 0x13e4ae530>
    Calendar Year: 2016
    Month: 2
    Leap month: no
    Day: 15
    Hour: 8
    Minute: 0
    Second: 0
    Quarter: 0
    Year for Week of Year: 2016
    Week of Year: 7

(lldb) 

(lldb) po ISO8601.timeZone
Local Time Zone (Asia/Shanghai (GMT+8) offset 28800)

(lldb) po date
2016-02-15 16:00:00 +0000

(lldb) po [ISO8601 components:unit fromDate:date];
<NSDateComponents: 0x13ec15150>
    Calendar Year: 2016
    Month: 2
    Leap month: no
    Day: 16
    Hour: 0
    Minute: 0
    Second: 0
    Quarter: 0
    Year for Week of Year: 2016
    Week of Year: 7

(lldb) 

同样，在不重新启动应用程序的情况下，将系统时区改回GMT-8，现在奇怪的事情发生了，GMT-8时区的

Day

值是

16

，这是不正确的！我错过了什么？请帮忙

(lldb) po ISO8601.timeZone
Local Time Zone (America/Los_Angeles (GMT-8) offset -28800)

(lldb) po date
2016-02-15 16:00:00 +0000

(lldb) po [ISO8601 components:unit fromDate:date];
<NSDateComponents: 0x13f888260>
    Calendar Year: 2016
    Month: 2
    Leap month: no
    Day: 16
    Hour: 0
    Minute: 0
    Second: 0
    Quarter: 0
    Year for Week of Year: 2016
    Week of Year: 7

(lldb) 

（lldb）po ISO8601.0时区
当地时区（美国/洛杉矶（GMT-8）偏移量-28800）
（lldb）采购订单日期
2016-02-15 16:00:00 +0000
（lldb）采购订单[ISO8601组件：单位起始日期：日期]；
日历年：2016年
月份：2
闰月：没有
日期：16
小时：0
分钟:0
秒：0
季度：0
年度每周的年度：2016年
年内星期：7
（lldb）