iOS-經過日期計算是周幾

//經過日期求星期
- (NSString*)fromDateToWeek:(NSString*)selectDate
{
    NSInteger yearInt = [selectDate substringWithRange:NSMakeRange(0, 4)].integerValue;
    NSInteger monthInt = [selectDate substringWithRange:NSMakeRange(4, 2)].integerValue;
    NSInteger dayInt = [selectDate substringWithRange:NSMakeRange(6, 2)].integerValue;
    int c = 20;//世紀
    NSInteger y = yearInt -1;//年
    NSInteger d = dayInt;
    NSInteger m = monthInt;
    int w =(y+(y/4)+(c/4)-2*c+(26*(m+1)/10)+d-1)%7;
    NSString *weekDay = @"";
    switch (w) {
        case 0:
            weekDay = @"週日";
            break;
        case 1:
            weekDay = @"週一";
            break;
        case 2:
            weekDay = @"週二";
            break;
        case 3:
            weekDay = @"週三";
            break;
        case 4:
            weekDay = @"週四";
            break;
        case 5:
            weekDay = @"週五";
            break;
        case 6:
            weekDay = @"週六";
            break;
        default:
            break;
    }
    return weekDay;
}