I experienced what are often called "bipolar blackouts" or "manic blackouts". Google these terms to learn more. I don't even remember perhaps four or even five of my hospitalizations, or more than flashes of them. Severe depression can also be so cognitively imparing that time seems to disappear, especially if the time was spent inactive to an almost paralyzing degree. As for depressions, though I can't speak for everyone's experience, I have the ability to easily forget, in a sense or to a great degree, the pain or numbness of those states when stability returns. Mixed episodes or frightening psychosis, on the other hand, is a different story for me. Or if I don't remember some of it, I believe the trauma of some have had negative effects on me later.

I, too, have had a couple dissociative amnesia periods, but mine were extremely brief ( minutes).