iOSアプリで「〜日前」表示

対象日時のunixタイムスタンプをを渡すと
「〜日前」「〜時間前」「〜分前」表示に変換

- (NSString *)_displayDate:(int)timestamp
{
    NSDate *now = [NSDate date];
    NSDate *date = [NSDate dateWithTimeIntervalSince1970:(NSTimeInterval)timestamp];
    NSTimeInterval passed = [now timeIntervalSinceDate:date];
    int min = passed / 60;
    if (min <= 1) {
        return @"1分前";
    }
    if (min < 60) {
        return [NSString stringWithFormat:@"%d分前", min];
    }
    int hour = min / 60;
    if (hour < 24) {
        return [NSString stringWithFormat:@"%d時間前", hour];
    }
    int day = hour / 24;
    return [NSString stringWithFormat:@"%d日前", day];
}