There is no requirement that your passport must have X months of validity, except that the passport must be valid (for your stay) , if you don't need a visa to visit Japan.

The standard tourist stay is 90 days. If your passport is due to expire in less than 90 days at time of landing, you'll spend a little more time at the immigration counter as the officer must manually adjust the date on the stamp to shorten the number of days you are allowed to stay.

If Timatic doesn't say 6 month's validity(remains silent), that means there is no such requirement. There is also iatatravelcentre.com where you can enter the passport expiry date and it will tell you you are fine.