If I left click or right click just once in the agenda/calendar a dialogbox opens. I think this should be (IMHO);
left click once -> nothing. This should make it easier to drag and select a time.
left click, drag -> open dialog with selected time.
double left click -> open dialog box. This should be default since it's default in every software i worked with.
right click once -> zoom in, for example ;from month to week, day, hour. Could be in a popup menu.
right click once on apointment-> delete, add note (or whatever, a popup menu with some standard features)
I have some other idea's but I want to see if this gets picked up.