This from Wikipedia

Horyu-ji (lit. Temple of the Flourishing Law) is a Buddhist temple in Ikaruga, Nara Prefecture, Japan. Its full name is Gakumonji or Learning Temple of the Flourishing Law, named as such because the site serves as a seminary as well as a monastery. The temple is widely acknowledged to have some of the oldest wooden buildings existing in the world.

Pagoda
The five-story pagoda stands at 32.45 meters in height (122 feet) and is widely regarded as one of the two oldest wood buildings in the world. It is likely of Baekje Korean or Tang Chinese style and was probably completed in 700. [26]. The brackets and railings of the pagoda are almost identical to those of the kondo. [27]. Of note is the probable existence of a reliquary or sarira casket underneath the pagoda. The massive foundation stone, set three meters into the ground, contains a hollow for relics. [28]. Unfortunately, because of the weight of the pagoda itself, the treasures will probably never be retrieved. [29]. Visitors are not allowed to enter the pagoda and far from having a function is designed to inspire people.

And this article suggests the pagoda is even older, built in the 7th century ...

100 YEARS OLDER THAN SUPPOSED?
World Heritage Pagoda

March 29, 2001
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The five-storied pagoda of the Horyuji, a Buddhist temple on the outskirts of the ancient capital of Nara, is the oldest surviving wooden structure in the world and is on the UNESCO World Heritage list. Most Japanese have visited Horyuji at least once in their lives, often on a middle- or high-school study trip. Recently historians have been engaged in a lively debate over when the pagoda was constructed. Many people learned from their history textbooks that it was rebuilt at the beginning of the eighth century. However, new research using x-ray photography is giving rise to a suspicion that it was actually built 100 years earlier than this.

Core Timber Felled in A.D. 594
The controversy has arisen because a recent scientific examination of the shinbashira, the "heart post" that passes through the center of the pagoda, showed that the hinoki (Japanese cypress) wood used for this post was felled in A.D. 594. Assuming this timber was used shortly after it was felled, it means that the construction of the pagoda took place not at the beginning of the eighth century (around 711), as is generally believed, but about a century earlier.

The generally held theory has it that Horyuji, including the pagoda, was first built around 607 by Prince Shotoku, the prince regent. These structures were burnt down in 670, and the temple was rebuilt some 100 meters away from the original site, being completed by around 711. The buildings rebuilt then constitute the present Sai-in, the western precinct of the temple. As the oldest surviving wooden buildings in the world, the quality of their construction is recognized by specialists throughout the world. In spite of the fact that the structure consists almost entirely of interlocking pieces of wood, the five-story pagoda has not succumbed to earthquakes, even though Japan is in a major earthquake zone.

There are several grounds for believing that the temple was destroyed and rebuilt. In the first place, the Nihon Shoki (Chronicles of Japan), a historical work compiled in 720, states unequivocally that it was "burnt down." Furthermore, the original site of the temple, showing evidence of having been involved in a fire, was discovered in the 1930s about 100 meters to the southeast of the Sai-in. In addition, according to the Nara National Cultural Properties Research Institute, the roof tiles of the present temple buildings date from the latter half of the seventh century and later. None date from an earlier period. These considerations led to the acceptance of the reconstruction theory.

"Burnt and Rebuilt" Theory Focus of Debate Again
However, if the cypress wood felled in 594 was used for the shinbashira of the pagoda immediately after, and it is still there, the theory becomes doubtful. Various explanations for this anomaly have been put forward by archaeologists and others. One is that the present shinbashira originated from another temple and was reused for the Horyuji pagoda. Another is that the timber was stored unused for an extended period.

The felling of the cypress was dated to 594 on the basis of research by the Nara institute. During the period from 1943 to 1954, while the pagoda was dismantled for repairs, a piece of the shinbashira about 80 centimeters in diameter and 10 centimeters thick was removed and put in storage at Kyoto University. The Nara institute announced in February 2001 that tests it had carried out using a combination of X-ray photography and dendrochronology (the dating of wood by examining the sequence of annual growth-ring widths) had established that the wood came from a tree felled in 594. (A similar examination 15 years earlier, conducted without the benefit of X-ray photography, was unable to establish an accurate date.)

This firm date for the felling of the tree used for the shinbashira raised the possibility that the Horyuji--one of Japan's most important ancient monuments--may have been constructed earlier than previously thought. Depending on the results of future research, Japanese people may have to relearn one of the key dates in the nation's architectural history.

More info:
http://web-japan.org/trends00/honbun/tj010330.html