iOS日期处理

//转换创建时间的格式    NSDateFormatter *fmt = [[NSDateFormatter alloc] init];    fmt.dateFormat = @"EEE MMM dd HH:mm:ss Z yyyy";    fmt.locale = [NSLocale localeWithLocaleIdentifier:@"en_US"];    NSDate *createdDate = [fmt dateFromString:_created_at];    //NSDate *createdDate = [fmt dateFromString:@"Thu May 19 13:20:20 +0800 2014"];    fmt.dateFormat = @"yyyy-MM-dd HH:mm:ss";    NSString *createdStr = [fmt stringFromDate:createdDate];    createdDate = [fmt dateFromString:createdStr];    NSDate *now = [NSDate date];//2015-05-22 14:16:16 +0000    NSCalendar *calender = [NSCalendar currentCalendar];    //why?    NSDateComponents *compareComponents = [calender components:(NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay |NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond ) fromDate:createdDate toDate:now options:0]; //!!!!!!!    //NSLog(@"%@",compareComponents);    NSDateComponents *createdComponents = [calender components:(NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay ) fromDate:createdDate];    NSDateComponents *nowComponents = [calender components:(NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay ) fromDate:now];    BOOL isThisYear = createdComponents.year == nowComponents.year;    BOOL isToday = createdComponents.month == nowComponents.month && createdComponents.day == nowComponents.day;    BOOL isYesterday = createdComponents.month == nowComponents.month && compareComponents.day == 1;    if (isThisYear) { //今年        if (isToday) {//今天            if (fabs([now timeIntervalSinceDate:createdDate]) < 60) {//小于一分钟                return @"刚刚";            } else if (compareComponents.hour == 0) { //小于一个小时                return [NSString stringWithFormat:@"%ld分钟前",compareComponents.minute];            } else { //大于一个小时                return [NSString stringWithFormat:@"%ld小时前",compareComponents.hour];            }        } else if (isYesterday) { //昨天            fmt.dateFormat = @"HH:mm:ss";            NSString *str = [fmt stringFromDate:createdDate];            return [NSString stringWithFormat:@"昨天 %@",str];        } else { //今年的其他日子            fmt.dateFormat = @"MM-dd HH:mm:ss";            return [fmt stringFromDate:createdDate];        }    } else {//其他年份        return [fmt stringFromDate:createdDate];    }