Quite a bit. I have a very bad memory for personal history anyway - I have a vague timeline of significant dates in my head, and a handful of random "vivid" memories, maybe one per year, that have been nailed down by neural happenstance. But if you asked me what I was doing yesterday evening, I think I would end up randomly selecting an evening from the last three or so - unless I painstakingly solved it in the manner of a logic puzzle ("I go to the gym on Wednesdays, and yesterday was Thursday, so I guess I was at the gym").