When the new Residence Management System in Japan went into effect last July, the Immigration bureau began issuing resident cards to foreigners but they didn’t require foreigners to immediately obtain them. The old alien card can be used as a valid substitute for a certain period of time. For most foreigners, that would be until their visa (period of stay) expires.
But what if your valid alien card is due to expire long before your visa expires? Do you have to replace it then with the new resident card? Of course, you can always just apply for a new resident card even though it is not yet required. Most busy people however would rather do it when it is really necessary in order to save time and money.
The English website of the Immigration Bureau does not mention anything about the validity period written on the alien card. It is in their Japanese pamphlet where I found useful information. Here is what it says:
外国人登録証明書上の「次回確認申請期間」の記載にかかわらず、上記のみなされる期間が、在留カード又は特別永住者証明書としての有効期間となります。
Here is my translation of the note:
Regardless of the "Next verification (renewal) application period" stated on the alien registration certificate, the above-mentioned period will be the effective period where [the alien card] is deemed as a resident card or a special permanent resident certificate.
In short, you can safely ignore the renewal date written on the alien card and just use it as a substitute until the time you are required to get a new resident card.