1.时间字符串的转化

//日期转时间
- (NSString *)dateToString:(NSDate *)date {
    NSDateFormatter* dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSString *dateString = [dateFormatter stringFromDate:date]; 
    return dateString;
}
//字符串转时间戳
- (NSTimeInterval)dateWithString:(NSString *)string {
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm"];
    NSDate *date = [dateFormatter dateFromString:string];
    return date.timeIntervalSince1970;
}

2.常见的时间转化

- (NSString *)formattedDateDescription
{
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    
    [dateFormatter setDateFormat:@"yyyy-MM-dd"];
    NSString *theDay = [dateFormatter stringFromDate:self];//日期的年月日
    NSString *currentDay = [dateFormatter stringFromDate:[NSDate date]];//当前年月日
    
    NSInteger timeInterval = -[self timeIntervalSinceNow];
    if (timeInterval < 60) {
        return @"1分钟内";
    } else if (timeInterval < 3600) {//1小时内
        return [NSString stringWithFormat:@"%ld分钟前", timeInterval / 60];
    } else if (timeInterval < 21600) {//6小时内
        return [NSString stringWithFormat:@"%ld小时前", timeInterval / 3600];
    } else if ([theDay isEqualToString:currentDay]) {//当天
        [dateFormatter setDateFormat:@"HH:mm"];
        return [NSString stringWithFormat:@"今天 %@", [dateFormatter stringFromDate:self]];
    } else if ([[dateFormatter dateFromString:currentDay] timeIntervalSinceDate:[dateFormatter dateFromString:theDay]] == 86400) {//昨天
        [dateFormatter setDateFormat:@"HH:mm"];
        return [NSString stringWithFormat:@"昨天 %@", [dateFormatter stringFromDate:self]];
    } else {//以前
        [dateFormatter setDateFormat:@"MM-dd HH:mm"];
        return [dateFormatter stringFromDate:self];
    }
}

3.已经知道当前时间，计算未来或者以前的时间

    NSCalendar *calendar = [NSCalendar currentCalendar];
    calendar.timeZone = [NSTimeZone localTimeZone];
    NSDateComponents *comp = [calendar components:(NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay) fromDate:date];
    //comp可以修改年月日等时间
    [comp setDay:1];
    NSDate *firstDayOfMonthDate = [calendar dateFromComponents:comp];

4.固定app的时区，不会根据位置改变(下面是东八区)

    [NSTimeZone setDefaultTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"GMT+0800"]];

更多时间资料：http://my.oschina.net/yongbin45/blog/156181