NSDateComponents 使用总结

1.用日历计算相差天数（默认会把时间也算上）如下：

//2017-03-20 15:58:50  [NSDate date]
//2017-03-24 01:13:59  startDate
    NSDate *startTime = [NSDate dateWithTimeIntervalSince1970:1490289239];

NSCalendar *calendar = [NSCalendar currentCalendar];
    NSCalendarUnit unit = kCFCalendarUnitYear | kCFCalendarUnitMonth | NSCalendarUnitDay | kCFCalendarUnitHour | kCFCalendarUnitMinute | kCFCalendarUnitSecond;
    NSDateComponents *dateComponents = [calendar components:unit fromDate:[NSDate date] toDate:startDate options:0];
    NSLog(@"%@", dateComponents);

输出结果：
2017-03-20 15:58:48.722 111[14894:146007] Calendar Year: 0 Month: 0 Day: 3 Hour: 9 Minute: 15 Second: 10

为这个时候你就会发现，20跟24号，我们实际想要的结果是4天，但是这个得到的结果却是3天。解决方法如下：

NSCalendar *calendar = [NSCalendar currentCalendar];
    NSCalendarUnit unit = kCFCalendarUnitYear | kCFCalendarUnitMonth | NSCalendarUnitDay | kCFCalendarUnitHour | kCFCalendarUnitMinute | kCFCalendarUnitSecond;
    
    NSDateComponents *currentDateCompontes = [calendar components:unit fromDate:[NSDate date]];
    NSDateComponents *startDateCompontes = [calendar components:unit fromDate:startTime];
    
    NSDate *currentDate = [calendar dateFromComponents:[self transformDate:currentDateCompontes]];
    NSDate *startDate = [calendar dateFromComponents:[self transformDate:startDateCompontes]];
    
    NSDateComponents *dateComponents = [calendar components:unit fromDate:currentDate toDate:startDate options:0];
    NSLog(@"%@", dateComponents);

//将时分秒设置为00:00:00 只计算日期相差天数
- (NSDateComponents *)transformDate:(NSDateComponents *)currentDate;
{
    NSDateComponents *dateCompontes = [[NSDateComponents alloc] init];
    [dateCompontes setYear:currentDate.year];
    [dateCompontes setMonth:currentDate.month];
    [dateCompontes setDay:currentDate.day];
    [dateCompontes setHour:0];
    [dateCompontes setMinute:0];
    [dateCompontes setSecond:0];
    return dateCompontes;
}

2017-03-20 16:06:26.768 111[15157:150705] Calendar Year: 0 Month: 0 Day: 4 Hour: 0 Minute: 0 Second: 0
这样就和预想的结果一样了。

2.还有一种方法就是拿到日期，直接对天进行相减操作也可实现

NSCalendar *calendar = [NSCalendar currentCalendar];
    NSCalendarUnit unit = kCFCalendarUnitYear | kCFCalendarUnitMonth | NSCalendarUnitDay;
    
    NSDateComponents *currentDateCompontes = [calendar components:unit fromDate:[NSDate date]];
    NSDateComponents *startDateCompontes = [calendar components:unit fromDate:startTime];
    NSLog(@"%ld", startDateCompontes.day - currentDateCompontes.day);

2017-03-20 16:14:31.910 111[15433:155187] 4