During what time of the year can Centaurus be seen from Tokyo? During exactly what dates of the year can the Centaurus constellation be seen from Tokyo, Japan? And what is the ideal date for viewing it from Tokyo?  I found on a website that it can been seen May-June in New Zealand, best visible at 21:00 (9pm) during May and that the midline is May 20th.

(According to the Japanese Wikipedia page for Centaurus, only from Okinawa can the entire constellation be seen; in Tokyo, apparently only part of it can be seen.)
You can use an application that shows the stars at any time and location. Stellarium is free, has nice features and isn't too complicated. You can select a star/constellation and flip through the time of day and through the days to see how its elevation changes over time. 

In general, everyone with the same latitude sees the same sky at a certain date (assuming an equal elevation, which counts if you're standing on Mt. Fuji). So those dates for New Sealand are irrelevant for you. 

Using the StarmapPro application, I made the following chart for your location (Tokyo) showing the elevation of Centaur at midnight for each month. 



As you can see, it's visible from halfway march until the first half of may, with its maximum around the first week of april. It comes barely above the horizon, so you'll need a clear view to the horizon and try to avoid light pollution (good luck with that in Tokyo ;-)