NSDate添加常用Category，用于得到是否是今天，昨天等

1、使用说明

    NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
    fmt.dateFormat = @"yyyy-MM-dd HH:mm:ss.S";
    NSDate *createDate = @"从接口中得到的时间";
    if (createDate.isThisYear) {
        if (createDate.isToday) { // 今天
            NSDateComponents *cmps = [createDate deltaWithNow];
            if (cmps.hour >= 1) { // 至少是1小时前发的
                status.created_at =  [NSString stringWithFormat:@"%ld小时前", (long)cmps.hour];
            } else if (cmps.minute >= 1) { // 1~59分钟之前发的
                status.created_at = [NSString stringWithFormat:@"%ld分钟前", (long)cmps.minute];
            } else { // 1分钟内发的
                status.created_at = @"刚刚";
            }
        } else if (createDate.isYesterday) { // 昨天
            fmt.dateFormat = @"昨天 HH:mm";
            status.created_at = [fmt stringFromDate:createDate];
        } else { // 至少是前天
            fmt.dateFormat = @"MM-dd HH:mm";
            status.created_at = [fmt stringFromDate:createDate];
        }
    } else { // 非今年
        fmt.dateFormat = @"yyyy-MM-dd";
        status.created_at = [fmt stringFromDate:createDate];
    }

2、实现细节
NSDate+Estension.h文件

@interface NSDate (Estension)
/**
 *  是否为今天
 */
- (BOOL)isToday;
/**
 *  是否为昨天
 */
- (BOOL)isYesterday;
/**
 *  是否为今年
 */
- (BOOL)isThisYear;

/**
 *  返回一个只有年月日的时间
 */
- (NSDate *)dateWithYMD;

/**
 *  获得与当前时间的差距
 */
- (NSDateComponents *)deltaWithNow;
@end

NSDate+Estension.m文件

@implementation NSDate (Estension)
/**
 *  是否为今天
 */
- (BOOL)isToday
{
    NSCalendar *calendar = [NSCalendar currentCalendar];
    int unit = NSCalendarUnitDay | NSCalendarUnitMonth |  NSCalendarUnitYear;

    // 1.获得当前时间的年月日
    NSDateComponents *nowCmps = [calendar components:unit fromDate:[NSDate date]];

    // 2.获得self的年月日
    NSDateComponents *selfCmps = [calendar components:unit fromDate:self];
    return
    (selfCmps.year == nowCmps.year) &&
    (selfCmps.month == nowCmps.month) &&
    (selfCmps.day == nowCmps.day);
}

/**
 *  是否为昨天
 */
- (BOOL)isYesterday
{
    // 2014-05-01
    NSDate *nowDate = [[NSDate date] dateWithYMD];

    // 2014-04-30
    NSDate *selfDate = [self dateWithYMD];

    // 获得nowDate和selfDate的差距
    NSCalendar *calendar = [NSCalendar currentCalendar];
    NSDateComponents *cmps = [calendar components:NSCalendarUnitDay fromDate:selfDate toDate:nowDate options:0];
    return cmps.day == 1;
}

- (NSDate *)dateWithYMD
{
    NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
    fmt.dateFormat = @"yyyy-MM-dd";
    NSString *selfStr = [fmt stringFromDate:self];
    return [fmt dateFromString:selfStr];
}

/**
 *  是否为今年
 */
- (BOOL)isThisYear
{
    NSCalendar *calendar = [NSCalendar currentCalendar];
    int unit = NSCalendarUnitYear;

    // 1.获得当前时间的年月日
    NSDateComponents *nowCmps = [calendar components:unit fromDate:[NSDate date]];

    // 2.获得self的年月日
    NSDateComponents *selfCmps = [calendar components:unit fromDate:self];

    return nowCmps.year == selfCmps.year;
}

- (NSDateComponents *)deltaWithNow
{
    NSCalendar *calendar = [NSCalendar currentCalendar];
    int unit = NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;
    return [calendar components:unit fromDate:self toDate:[NSDate date] options:0];
}
@end