为什么 [calendar dateFromComponents:] 总是 return 小时 16？

Question

当我将 [NSDate date] 传递给 NSDateComponents ，然后在修改工作日后传回时，我总是得到小时 16:00:00 。为什么？

代码如下：

     {
        NSCalendar *calendar = [[NSCalendar alloc]initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
        NSDate *now = [NSDate date];
        NSLog(@"now = %@",now);

        NSDateComponents *firstDayOfWeek = [calendar components:NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitWeekday fromDate:now];
        firstDayOfWeek.weekday = 1; // Monday as first day of week

        NSDate *firstDayOfWeekDate = [calendar dateFromComponents:firstDayOfWeek];
        NSLog(@"first day = %@", firstDayOfWeekDate);
    }

    {
        NSCalendar *calendar = [[NSCalendar alloc]initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
        NSDate *now = [NSDate date];
        NSLog(@"now = %@",now);

        NSDateComponents *firstDayOfWeek = [calendar components:NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitWeekday | NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond fromDate:now];
        firstDayOfWeek.weekday = 1; // Monday as first day of week
        firstDayOfWeek.hour = 1;
        firstDayOfWeek.minute = 1;
        firstDayOfWeek.second = 1;

        NSDate *firstDayOfWeekDate = [calendar dateFromComponents:firstDayOfWeek];
        NSLog(@"first day = %@", firstDayOfWeekDate);
    }

输出为：

 now = 2016-01-18 03:14:08 +0000
 first day = 2016-01-17 16:00:00 +0000
 now = 2016-01-18 03:14:08 +0000
 first day = 2016-01-17 17:01:01 +0000

为什么小时从 16 开始？

Answer 1

这是由于时区的原因。

尝试在使用NSLog输出日期之前添加NSDateFormatter，例如

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setLocale:[NSLocale currentLocale]];
[dateFormatter setDateStyle:NSDateFormatterShortStyle];
[dateFormatter setTimeStyle:NSDateFormatterLongStyle];
NSString *dateDisplay = [dateFormatter stringFromDate:firstDayOfWeekDate];
NSLog(@"first day = %@",dateDisplay);

更多示例可以参考this article。

Answer 2

NSDateComponents 没有达到 16:00:00。现在是您当前时区的午夜。但是 NSLog 显示的是等效的 GMT 时间。这就是 +0000 的意思：例如格林威治标准时间 +00:00。

最重要的是，如果您想在本地时区查看生成的 NSDate 对象，您应该使用 NSDateFormatter.

为什么 [calendar dateFromComponents:] 总是 return 小时 16？

Why [calendar dateFromComponents:] always return hour 16 ?

nsdate

ios