How the renewal window works
If your period of stay is 6 months or longer, the Immigration Services Agency accepts an Application for Extension of Period of Stay (在留期間更新許可申請) from about 3 months before it ends. Shorter periods can be renewed any time before expiry.
Apply by the expiry date. If you do, the special period (特例期間) lets you stay — and keep working within your permission — until the decision or 2 months after expiry, whichever comes first. Online applications cannot be filed on the last day itself.
March, April, September and October are the busiest months at immigration offices; applying early avoids long lines and leaves time for extra documents.