昨天今天明天

NSDate * date = [NSDate date];//当前时间
NSDate *lastDay = [NSDate dateWithTimeInterval:-24*60*60 sinceDate:date];//前一天
NSDate *nextDay = [NSDate dateWithTimeInterval:24*60*60 sinceDate:date];//后一天

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"YYYY-MM-dd"];
NSString *dateString = [dateFormatter stringFromDate:currentDate];