Can I use the Tokyo Skytree as clock? After my observation today at the Skytree in Tokyo, I am wondering if this building could be used as a giant sundial? 

As an aside, it would be interesting to consider how the Skytree might be used to display the time.
Anything which casts a shadow can become a sundial, as JH's comment points out.

The other thing you need to know is the latitude of the building, as that affects the change in shadow direction per hour of elapsed time.  Worse, due to the eccentricity of Earth's orbit (and rotation axis), there's an offset between clock time and sundial time which varies over the year.  You can get most of the formulas at the Wikipedia page .

There's a handy calculator at blocklayer.com