iOS 日期与字符串格式互转

- (void)viewDidLoad {
    [super viewDidLoad];

    NSDate *nowDate = [NSDate date];
    NSString *str = [self dateToString:nowDate withDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSLog(@"%@",str);

    NSDate *newdate = [self stringToDate:@"2016-01-18 15:13:12" withDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSLog(@"%@",newdate);

}

//日期格式转字符串
- (NSString *)dateToString:(NSDate *)date withDateFormat:(NSString *)format
{
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:format];
    NSString *strDate = [dateFormatter stringFromDate:date];
    return strDate;
}

//字符串转日期格式
- (NSDate *)stringToDate:(NSString *)dateString withDateFormat:(NSString *)format
{
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:format];

    NSDate *date = [dateFormatter dateFromString:dateString];
    return [self worldTimeToChina:date];
}

//将世界时间转化为中国区时间
- (NSDate *)worldTimeToChinaTime:(NSDate *)date
{
    NSTimeZone *timeZone = [NSTimeZone systemTimeZone];
    NSInteger interval = [timeZone secondsFromGMTForDate:date];
    NSDate *localeDate = [date  dateByAddingTimeInterval:interval];
    return localeDate;
}

///< 获取当前时间的: 前一周(day:-7)丶前一个月(month:-30)丶前一年(year:-1)的时间戳
+ (NSString *)ddpGetExpectTimestamp:(NSInteger)year month:(NSUInteger)month day:(NSUInteger)day {
 
    ///< 当前时间
    NSDate *currentdata = [NSDate date];
 
    ///< NSCalendar -- 日历类，它提供了大部分的日期计算接口，并且允许您在NSDate和NSDateComponents之间转换
    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    
    /*
    ///<  NSDateComponents：时间容器，一个包含了详细的年月日时分秒的容器。
    ///< 下例：获取指定日期的年，月，日
    NSDateComponents *comps = nil;
    comps = [calendar components:NSCalendarUnitYear|NSCalendarUnitMonth|NSCalendarUnitDay fromDate:currentdata];
    NSLog(@"年 year = %ld",comps.year);
    NSLog(@"月 month = %ld",comps.month);
    NSLog(@"日 day = %ld",comps.day);*/
 
    
    NSDateComponents *datecomps = [[NSDateComponents alloc] init];
    [datecomps setYear:year?:0];
    [datecomps setMonth:month?:0];
    [datecomps setDay:day?:0];
    
    ///< dateByAddingComponents: 在参数date基础上，增加一个NSDateComponents类型的时间增量
    NSDate *calculatedate = [calendar dateByAddingComponents:datecomps toDate:currentdata options:0];
    
    ///< 打印推算时间
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat:@"yyyy-MM-dd hh:mm:ss"];
    NSString *calculateStr = [formatter stringFromDate:calculatedate];
    
    DDLog(@"calculateStr 推算时间: %@",calculateStr );
 
    ///< 预期的推算时间
    NSString *result = [NSString stringWithFormat:@"%ld", (long)[calculatedate timeIntervalSince1970]];
    
    return result;
}

//获取日期date对应月的第一天日期
+ (NSString *)getMonthFirstDayWithDate:(NSDate *)date format:(NSString *)aformat;
//获取日期date对应月的最后一天日期
+ (NSString *)getMonthLastDayWithDate:(NSDate *)date format:(NSString *)aformat;
//获取对应日期是周几
+ (NSInteger)getWeekDayFromDate:(NSDate *)date;


+ (NSString *)getMonthFirstDayWithDate:(NSDate *)date format:(NSString *)aformat{
    NSDate *newDate = date;
    double interval = 0;
    NSDate *firstDate = nil;
    NSCalendar *calendar = [NSCalendar currentCalendar];
    BOOL bl = [calendar rangeOfUnit:NSCalendarUnitMonth startDate:& firstDate interval:&interval forDate:newDate];
    if (bl) {
        NSDateFormatter *myDateFormatter = [[NSDateFormatter alloc] init];
        [myDateFormatter setDateFormat:aformat];
        NSString *firstString = [myDateFormatter stringFromDate: firstDate];
        return firstString;
    }
    return @"";
}
+ (NSString *)getMonthLastDayWithDate:(NSDate *)date format:(NSString *)aformat{
    NSDate *newDate = date;
    double interval = 0;
    NSDate *firstDate = nil;
    NSDate *lastDate = nil;
    NSCalendar *calendar = [NSCalendar currentCalendar];
    BOOL bl = [calendar rangeOfUnit:NSCalendarUnitMonth startDate:& firstDate interval:&interval forDate:newDate];
    if (bl) {
        lastDate = [firstDate dateByAddingTimeInterval:interval - 1];
        NSDateFormatter *myDateFormatter = [[NSDateFormatter alloc] init];
        [myDateFormatter setDateFormat:aformat];
        NSString *lastString = [myDateFormatter stringFromDate: lastDate];
        return lastString;
    }
    return @"";
}

+ (NSInteger)getWeekDayFromDate:(NSDate *)date{
    NSArray *tempWeek = @[@"7",@"1",@"2",@"3",@"4",@"5",@"6"];
    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    
    NSDateComponents *comps = [[NSDateComponents alloc] init];
    
    NSInteger unitFlags = NSCalendarUnitYear |NSCalendarUnitMonth | NSCalendarUnitDay |NSCalendarUnitWeekday | NSCalendarUnitHour |NSCalendarUnitMinute |NSCalendarUnitSecond;
    
    comps = [calendar components:unitFlags fromDate:date];
    //  1、2、3、4、5、6、7 分别对应 周日、周一、周二、周三、周四、周五、周六
    NSInteger week = [comps weekday];
    NSLog(@"---%ld",week);
  //  调整后 1 代表 周一
    return  [tempWeek[week-1] integerValue] ;
    
}

原文链接：https://blog.csdn.net/qq_28379951/article/details/79207385
原文链接：https://blog.csdn.net/u010545480/article/details/50538114
原文链接：https://www.jianshu.com/p/c89564c25e81