Time travelling would affect the future/past, you just wouldn't know it.
Unless someone starts bleating on about that very convenient alternative worldines mallarky.
Story: met this chap online who said he had travelled back to 1900ad, he reaconed that he had gone to some town in Austria, and spoken to the locals who thought he was a tourist, he noticed a young lad who was great at art, spoke to this lad and found out he was called Adolf, the lad was 11yrs old and wanted to be a monk. Anyway, before I ramble on, the guy reacons he told the lad to pursue his art and ignore advice given, so he could follow his dreams instead.
The chap said to me that because of him, Hitler became the man we know today, and not some forgotten monk somewhere.
How true that story is I can't say, whether I believe it or not I won't say. But if it is true then timetravellers may be affecting our past/future and we will never know.