日期比较

+ (NSString *)compareDate:(NSDate *)date {    NSTimeInterval secondsPerDay = 24 * 60 * 60;        NSDate *today = [[NSDate alloc] init];    NSDate *yesterday;        yesterday = [today dateByAddingTimeInterval: -secondsPerDay];        // 10 first characters of description is the calendar date:    NSString * todayString = [[today description] substringToIndex:10];    NSString * yesterdayString = [[yesterday description] substringToIndex:10];        NSString * dateString = [[date description] substringToIndex:10];        if ([dateString isEqualToString:todayString]) {        return @"今天";    } else if ([dateString isEqualToString:yesterdayString]) {        return @"昨天";    } else {        NSDateFormatter *formatter = [[NSDateFormatter alloc] init];        formatter.dateFormat = @"MM月dd日";        return [formatter stringFromDate:date];    }}