JTB Sunrise tours

I am planning to join the JTB tours ( those half-day tours) and was planning to book it thru a travel agent. My first question:

1. Can I book using other ways? Will it be available in hotels in Japan, etc?

2. Also, if you set a date, is that the only date you can join the tours or can you join another day if you cant make it?

You can book at the tour desk in greater hotels in Tokyo, check with the hotel you intend to stay. But you can book online, too, see following webside, what shows tour conditions, too:
http://www.jtbgmt.com/sunrisetour/index.aspx

I am also considering taking tours and I found Japan Gray Line Tour. Check them out and see if you like their tours also before buying something from Sunrise tours. It is always good to have options to choose from :)
http://www.jgl.co.jp/inbound/index.htm