iOS 计算两个日期之前相隔多少晚

iOS的NSCalendar可以计算两个日期之间相隔的天数，代码如下：

- (NSInteger)daysFromDate:(NSDate *)startDate toDate:(NSDate *)endDate{
NSLog(@"start:%@", startDate);
NSLog(@"end:%@", endDate);
NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

unsigned int unitFlags = NSDayCalendarUnit;
NSDateComponents *comps = [gregorian components:unitFlags fromDate:startDate  toDate:endDate  options:0];
NSLog(@"day:%d", [comps day]);
return [comps day];
}

但这样得到的结果有时会出现下面的现象，

start:2014-01-19 04:15:22 +0000

end:2014-01-20 04:15:22 +0000

day:1

start:2014-01-19 04:15:44 +0000

end:2014-01-21 04:15:33 +0000

day:1

在stackoverflow上找到了一个可行的方法，经稍微改进，现在可以只计算日期，忽略时间，得到两个日期之间相差多少晚，代码如下：

- (NSInteger)daysFromDate:(NSDate *)startDate toDate:(NSDate *)endDate{
NSDateFormatter *temp = [[NSDateFormatter alloc] init];
[temp setDateFormat:@"yyyy-MM-dd"];
NSDate *stDt = [temp dateFromString:[temp stringFromDate:startDate]];
NSDate *endDt =  [temp dateFromString:[temp stringFromDate:endDate]];

NSTimeZone *zone = [NSTimeZone systemTimeZone];
NSInteger interval = [zone secondsFromGMTForDate:stDt];
stDt = [stDt dateByAddingTimeInterval: interval];

interval = [zone secondsFromGMTForDate:endDt];
endDt = [endDt dateByAddingTimeInterval:interval];

unsigned int unitFlags = NSMonthCalendarUnit | NSDayCalendarUnit;
NSDateComponents *comps = [self.calendar components:unitFlags fromDate:stDt  toDate:endDt  options:0];

return [comps day];
}