以下NSDate的分类中写  
//  NSDate+Extension.m
 #import "NSDate+Extension.h"
@implementation NSDate (Extension)
/**
 *  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;
}

/**
 
 *  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;
}

/**
 
 *  返回一个只有年月日的时间
 
 */
- (NSDate *)dateWithYMD {
    
    NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
    
    fmt.dateFormat = @"yyyy-MM-dd";
    
    NSString *selfStr = [fmt stringFromDate:self];
    
    return [fmt dateFromString:selfStr];
}
/**
 
 *  获得与当前时间的差距
 
 */
- (NSDateComponents *)deltaWithNow {
    
    NSCalendar *calendar = [NSCalendar currentCalendar];
    
    int unit = NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;
    
    return [calendar components:unit fromDate:self toDate:[NSDate date] options:0];
}
/*
 为星期几
 */
- (NSString *)dateWithWeek {
    NSArray *weekday = [NSArray arrayWithObjects: [NSNull null], @"星期日", @"星期一", @"星期二", @"星期三", @"星期四", @"星期五", @"星期六", nil];
    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    NSDateComponents *components = [calendar components:NSWeekdayCalendarUnit fromDate:self];
    
    return [weekday objectAtIndex:components.weekday];
}

@end