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NSDate专家:我的计算方法无法生成一致的日期。为什么?

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just_another_coder · 技术社区 · 15 年前

我发明了一种方法来找到每个月的第三个星期五。

当我从东部时区运行时,一切都很好。

当我从中环查的时候,日期取消了。

我的方法:

-(NSDate*) getSaturdayAfterThirdFriday:(NSDate*)date {
    NSDate* resultDate = nil;

    if( date != nil ) {

        NSDateFormatter *df = [[NSDateFormatter alloc] init];

        // start at the 1st of the month
        NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

        NSTimeZone *zone = [NSTimeZone timeZoneWithName:@"EST"];
        [calendar setTimeZone:zone];
        NSDateComponents *comp = [calendar components:(NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit) fromDate:date];
        [comp setDay:1];
        date= [calendar dateFromComponents:comp];
        [calendar release];

        // count number of 'fri' (including today)
        BOOL foundFlag = NO;
        NSString* dateString = nil;
        NSInteger count = 0;

        [df setDateFormat:@"EEE"];

        while (foundFlag == NO) {
            dateString = [df stringFromDate:date];
            if( [dateString caseInsensitiveCompare:@"fri"] == 0 ) {
                count++;
                NSLog(@"Found Friday: %@", [date description]);
            }
            if( count >= 3 ) foundFlag = YES;
            else date = [NSDate dateWithTimeInterval:86400 sinceDate:date];
        }

        // get the next day (Saturday)
        resultDate = [NSDate dateWithTimeInterval:86400 sinceDate:date];

        [df release];
    }

    return resultDate;
}

使用此方法创建传递的日期:

-(NSDate*) getDateForEasternTime:(NSDate*)date {
    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    NSTimeZone *zone = [NSTimeZone timeZoneWithName:@"EST"];
    [calendar setTimeZone:zone];
    NSDateComponents *comp = [calendar components:(NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit) fromDate:date];
    date = [calendar dateFromComponents:comp];
    [calendar release];
    NSLog(@"getDateForEasternTime: %@", [date description]);
    return date;
}

我只需要根据东部时间来计算日期。

这是我在东部时区的日志输出:

getDateForEasternTime: 2010-11-14 05:00:00 GMT

Found Friday: 2010-11-05 05:00:00 GMT

Found Friday: 2010-11-12 05:00:00 GMT

Found Friday: 2010-11-19 05:00:00 GMT

对于中央时间:

getDateForEasternTime: 2010-11-14 05:00:00 GMT

Found Friday: 2010-11-05 05:00:00 GMT

Found Friday: 2010-11-13 05:00:00 GMT

Found Friday: 2010-11-20 05:00:00 GMT

更奇怪的是,如果我在时区往西走得更远,就会有差异。如果我在时区向东走,那里没有。

有谁能帮上忙吗?

编辑:

当我提到更改时区时,我指的是更改设备本身的时区。台词:

NSTimeZone *zone = [NSTimeZone timeZoneWithName:@"EST"];

保持不变。

1 回复 | 直到 15 年前

Sven 15 年前

你所有的代码都是多余的。使用 weekday 和 weekdayOrdinal 属性 NSDateComponents 您可以直接查询 NSCalendar 一个月的第三个星期五:

NSDateComponents *components = [[NSDateComponents alloc] init];
[components setWeekday:6];        // 1 = Sunday ... 7 = Saturday
[components setWeekdayOrdinal:3]; // 3rd Friday  
[components setMonth:11]; 
[components setYear:2010];
NSCalendar *currentCalendar = [NSCalendar currentCalendar];
NSDate *date = [currentCalendar dateFromComponents:components];

而且,在 Developer Documentation .