Just to clarify - I was talking about oldest surviving buildings as we were discussing time capsules.

There are archaeological sites where large amounts of construction had been carried out that are much older than 3500BC e.g. Çatalhöyük in Turkey:

http://en.wikipedia.org/wiki/%C3%87atalh%C3%B6y%C3%BCk

And, of course, Jericho:

http://en.wikipedia.org/wiki/Jericho