Until three or four years ago conditions like ME/CFS or Fibromyalgia were not recognized by the rules governing disability in Japan. It is a little known fact that the date of the first doctors visit can be the onset date, even if there was not a clear diagnosis for the disability. Even though the disability rules have changed in Japan, people with an ME/CFS diagnosis are still at a disadvantage. Like in the US, being aware of the nuances of the rules is the only way to even the odds.

Source: 【FP解説】年金の「知らないと損！」 病名の確定診断日が初診日？（ファイナンシャルフィールド） – Yahoo!ニュース