iOS 比较两个日期的大小

比较两个日期的大小（日期格式为yyyy-MM-dd HH:mm:ss）

+ (NSInteger)compareDate:(NSString*)aDate withDate:(NSString*)bDate
{
    NSInteger aa = 0;
    NSDateFormatter *dateformater = [[NSDateFormatter alloc] init];
    [dateformater setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSDate *dta = [[NSDate alloc] init];
    NSDate *dtb = [[NSDate alloc] init];
    
    dta = [dateformater dateFromString:aDate];
    dtb = [dateformater dateFromString:bDate];
    NSComparisonResult result = [dta compare:dtb];
    if (result==NSOrderedSame)
    {
        //相等
        aa = 0;
    }else if (result==NSOrderedAscending) {
        //bDate比aDate大
        aa = 1;
    }else if (result==NSOrderedDescending) {
        //bDate比aDate小
        aa = -1;
    }
    return aa;
}