NSDate的一个分类,判断是否是今天，昨天，今年,获得与当前时间的差距,返回一个只有年月日的日期

//
//  NSDate+YKD.h

#import <Foundation/Foundation.h>

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

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

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

#import "NSDate+YKD.h"

@implementation NSDate (YKD)
- (BOOL)isToday
{
//now: 2015-09-05 11:23:00
//self 调用这个方法的对象本身

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
NSDateComponents *selfCmps = [calendar components:unit fromDate:self];

return (selfCmps.year == nowCmps.year) && (selfCmps.month == nowCmps.month) && (selfCmps.day == nowCmps.day);
}
- (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;
}

- (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
NSDateComponents *selfCmps = [calendar components:unit fromDate:self];

return selfCmps.year == nowCmps.year;
}

/**
*  返回这种格式的日期 yyyy-MM-dd
*/
- (NSDate *)dateWithYMD
{
NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
fmt.dateFormat = @"yyyy-MM-dd";
NSString *selfStr = [fmt stringFromDate:self];
return [fmt dateFromString:selfStr];
}

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