iOS 时间和时间戳之间转化

//返回时间格式

NSCalendar  *calendar = [NSCalendar currentCalendar];

//1.获取当前的时间

NSDate *currentDate = [NSDate date];

// 获取年，月，日

NSDateComponents *components = [calendar components:NSCalendarUnitYear|NSCalendarUnitMonth|NSCalendarUnitDay fromDate:currentDate];

NSInteger currentYear = components.year;

NSInteger currentMonth = components.month;

NSInteger currentDay = components.day;

//2.获取消息发送时间

NSDate *msgDate = [NSDate dateWithTimeIntervalSince1970:timestamp/1000.0];

// 获取年，月，日

components = [calendar components:NSCalendarUnitYear|NSCalendarUnitMonth|NSCalendarUnitDay fromDate:msgDate];

CGFloat msgYead = components.year;

CGFloat msgMonth = components.month;

CGFloat msgDay = components.day;

//3.判断:

/*今天：(HH:mm)

*昨天: (昨天 HH:mm)

*昨天以前:（2015-09-26 15:27）

*/

NSDateFormatter *dateFmt = [[NSDateFormatter alloc] init];

if (currentYear == msgYead&& currentMonth == msgMonth&& currentDay == msgDay) {//今天 

dateFmt.dateFormat= @"hh:mm";

}else if(currentYear == msgYead&& currentMonth == msgMonth&& currentDay - 1 == msgDay){//昨天

dateFmt.dateFormat= @"昨天 hh:mm";

}else{//昨天以前

dateFmt.dateFormat= @"yyy-MM-dd hh:mm";

}