I don't believe a future employer would have to know unless you told them.

The question of my 'blank of time' in my resume did come up at an interview for a volunteer position. I acknowledged that yes I was out of work for a period of time and said only that I had dealt with a health issue which would have no impact on my ability to perform the volunteer duties. They are not allowed by law to ask what such a condition would be. I enjoy my new position and one day hope to graduate to a more regular position of employment.