iOS 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];
    }
}