和JAVA一样，IOS也有自己的日期格式。JAVA里面是SimpleDateFormat进行日期格式化输出。下面提供三个函数对日期的操作。

//利用字符串获取该字符串所表示日期的上一天
-(NSString*)upDate:(NSString*)nowdate{
    NSTimeInterval secondsPerDay = 24 * 60 * 60;
    NSDate *yesterday;
    yesterday=[[self convertDateFromString:nowdate] dateByAddingTimeInterval: -secondsPerDay];
    NSDateFormatter *f=[[NSDateFormatter alloc] init];
    [f setDateFormat:@"yyyy-MM-dd"];
    NSString *date=[NSString stringWithFormat:@"%@",[f stringFromDate:yesterday]];
    return date;
}
//利用字符串获取该字符串所表示日期的下一天
-(NSString*)downDate:(NSString*)nowdate{
    NSTimeInterval secondsPerDay = 24 * 60 * 60;
    NSDate *tomorrow;
    tomorrow=[[self convertDateFromString:nowdate] dateByAddingTimeInterval: secondsPerDay];
    NSDateFormatter *f=[[NSDateFormatter alloc] init];
    [f setDateFormat:@"yyyy-MM-dd"];
    NSString *date=[NSString stringWithFormat:@"%@",[f stringFromDate:tomorrow]];
    return date;
}
//将字符串转化为日期
-(NSDate*) convertDateFromString:(NSString*)uiDate
{
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init] ;
    [formatter setDateFormat:@"yyyy-MM-dd"];
    NSDate *date=[formatter dateFromString:uiDate];
    return date;
}

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