iPhone - 获取两个日期之间的天数

Question

我正在为 iPhone 编写 GTD 应用程序。对于到期任务，我想显示“明天到期”或“昨天到期”或“7 月 18 日到期”之类的内容。显然，即使距离任务还有不到 24 小时，我也需要显示“明天”(例如，用户在周六晚上 11 点查看并看到周日上午 8 点有任务)。因此，我编写了一个方法来获取两个日期之间的天数。这是代码...

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"yyyy-MM-dd-HH-mm"];

NSDate *nowDate = [dateFormatter dateFromString:@"2010-01-01-15-00"];
NSDate *dueDate = [dateFormatter dateFromString:@"2010-01-02-14-00"];

NSLog(@"NSDate *nowDate = %@", nowDate);
NSLog(@"NSDate *dueDate = %@", dueDate);

NSCalendar *calendar = [NSCalendar currentCalendar];

NSDateComponents *differenceComponents = [calendar components:(NSDayCalendarUnit)
                                                     fromDate:nowDate
                                                       toDate:dueDate
                                                      options:0];

NSLog(@"Days between dates: %d", [differenceComponents day]);

...这是输出:

NSDate *nowDate = 2010-01-01 15:00:00 -0700
NSDate *dueDate = 2010-01-02 14:00:00 -0700
Days between dates: 0

如您所见，该方法返回不正确的结果。它应该返回 1 作为两天之间的天数。我在这里做错了什么？

编辑:我写了另一种方法。我还没有进行广泛的单元测试，但到目前为止它似乎有效:

+ (NSInteger)daysFromDate:(NSDate *)fromDate inTimeZone:(NSTimeZone *)fromTimeZone untilDate:(NSDate *)toDate inTimeZone:(NSTimeZone *)toTimeZone {

    NSCalendar *calendar = [NSCalendar currentCalendar];
    unsigned unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit;

    [calendar setTimeZone:fromTimeZone];
    NSDateComponents *fromDateComponents = [calendar components:unitFlags fromDate:fromDate];

    [calendar setTimeZone:toTimeZone];
    NSDateComponents *toDateComponents = [calendar components:unitFlags fromDate:toDate];

    [calendar setTimeZone:[NSTimeZone defaultTimeZone]];
    NSDate *adjustedFromDate = [calendar dateFromComponents:fromDateComponents];
    NSDate *adjustedToDate = [calendar dateFromComponents:toDateComponents];

    NSTimeInterval timeIntervalBetweenDates = [adjustedToDate timeIntervalSinceDate:adjustedFromDate];
    NSInteger daysBetweenDates = (NSInteger)(timeIntervalBetweenDates / (60.0 * 60.0 * 24.0));

    NSDateComponents *midnightBeforeFromDateComponents = [[NSDateComponents alloc] init];
    [midnightBeforeFromDateComponents setYear:[fromDateComponents year]];
    [midnightBeforeFromDateComponents setMonth:[fromDateComponents month]];
    [midnightBeforeFromDateComponents setDay:[fromDateComponents day]];

    NSDate *midnightBeforeFromDate = [calendar dateFromComponents:midnightBeforeFromDateComponents];
    [midnightBeforeFromDateComponents release];

    NSDate *midnightAfterFromDate = [[NSDate alloc] initWithTimeInterval:(60.0 * 60.0 * 24.0)
                                                               sinceDate:midnightBeforeFromDate];

    NSTimeInterval timeIntervalBetweenToDateAndMidnightBeforeFromDate = [adjustedToDate timeIntervalSinceDate:midnightBeforeFromDate];
    NSTimeInterval timeIntervalBetweenToDateAndMidnightAfterFromDate = [adjustedToDate timeIntervalSinceDate:midnightAfterFromDate];

    if (timeIntervalBetweenToDateAndMidnightBeforeFromDate < 0.0) {

        // toDate is before the midnight before fromDate

        timeIntervalBetweenToDateAndMidnightBeforeFromDate -= daysBetweenDates * 60.0 * 60.0 * 24.0;

        if (timeIntervalBetweenToDateAndMidnightBeforeFromDate < 0.0)
            daysBetweenDates -= 1;
    }
    else if (timeIntervalBetweenToDateAndMidnightAfterFromDate >= 0.0) {

        // toDate is after the midnight after fromDate

        timeIntervalBetweenToDateAndMidnightAfterFromDate -= daysBetweenDates * 60.0 * 60.0 * 24.0;

        if (timeIntervalBetweenToDateAndMidnightAfterFromDate >= 0.0)
            daysBetweenDates += 1;
    }

    [midnightAfterFromDate release];

    return daysBetweenDates;
}

Answer 1

来自 components:fromDate:toDate:options: 的文档:

The result is lossy if there is not a small enough unit requested to hold the full precision of the difference.

由于差异小于一整天，因此它正确返回 0 天的结果。