This is a very tricky question to answer, indeed, as aspects of archaeological investigation have been progressively moving forward in time over the past decades. For, while for a long time, World War II remains were not considered of archaeological interest, they are these days. What's more, the most recent archaeological niche that I'm aware of is that of the archaeology of gaming, which brings the field into the 1980's, when game computers were starting to enter certain households.

As such, it's best to think of "the archaeological past" as a fluid and flexible concept, without clearly defined boundaries - a concept that moves forward in time with us to explore elements of our shared history that society takes an interest in, yet which course has previously not been put into writing and which developments are explored through the physical study of the material culture associated with it. This is to say that excavation is less of a defining characteristic of archaeology and its study of our past than is involvement of actual material relics/physical cultural traces, with the field differentiating itself from historical research exactly through involvement of the material record versus purely the written one. As such, one may even, as has been done by the Dutch national Museum of Antiquities, study the development of tooth brushes through the ages, up to the modern time.

It is important to keep in mind, however, that the concept of archaeology also wildly differs from country to country, with some considering it part of anthropology - that is, completely merging it with studies of cultures from the present-day - and others taking a more historical approach, relying more heavily on written records. The above perspective is one derived from the Dutch view on archaeology and may, furthermore, in true post-processualist manner, not even be a view held by all archaeological schools in the Netherlands - notwithstanding my personal belief that it does.

Le passé relevant de l'archéologie commence hier.

As an example take the ancient Minoans who started mining copper in Michigan, USA some 5,000 years before Christ and ended sharply around 1,300 BC.

The remains of their activities would be subject to archeological studies.

Copper mining in Michigan in "modern" times did not start until late 1800's by the early settlers (migrants from Europe).

Activities of the last episode would also be subject to archeological studies.

Zimbabwe was also a place of very early human activities that should be considered ( English word comes from the Greek "sidero" which means "iron" from red stain on a volcano near Athens) by trained archeologists.

Alexander's response aligns with how archaeologists in Australia would approach the question. If you think about archaeology as a way of investigating material culture - then you can apply the techniques of archaeology to studying the rubbish discarded by New Yorkers yesterday, for example, and what that rubbish tells us about American urban culture. From that perspective, the past can be as recent as yesterday. In other words, there is not a cutoff date after which it becomes inappropriate to use archaeology to study material culture.

For me archeology is already what happened yesterday. An example is studies of artifacts from World War II or the industrialization era. The concept of studying "yesterday"....

Archaeology is not a time span, but a method and Science. So there are many archaeologies as Classical, Prehistoric, Industrial, Under Water, Biblical, Egyptian Archaeology...