User blog comment:B214/General Discussion/@comment-85.154.16.59-20170708100042/@comment-9595671-20170708180437

If I recall right, the Japanese school year starts on April and ends in March, April being when its considered Spring in Japan and representing new beginnings to them.

Volume 18 is when its Christmas time, I need to check to be specific, but it has to be between January and March by then. So that's at least a 9 month minimum.